Worst game I've played (that I can think of ATM) The Urbz, probably the only sims spin-offs cause this one blows so much, you spend most of your time and money just running around trying to keep your needs fulfilled instead of actually getting things done. And most things you can pick up are hard to see, they're either too small or they blend into the background.

Best game I've played has to be Ut04, simply because no matter how often I play it or how long it is between times I play it I can still pick it up and have fun. (the same could be said for Oblivion but, since you need the DVD to play it, that kinda kills my ability to randomly play it)

There are far too many games that vie for my affection.

When I find a game I enjoy, I play it obsessively until I have done everything or damn near everything there is to do in that game.  The problem is that I tend to burn myself out before finishing the game.  Normally, I'd say the best of the best is the game I never grew weary of playing, but a lot of different factors obscure the accuracy of those results such as poor memory and/or events in my personal life.

So here's all of my favorites I can recall off-hand...

Best real-time RPG: Kingdom Hearts II / Kingdom Hearts I
Best turn-based RPG: Final Fantasy IV / Final Fintasy IX / Final Fantasy X-2
Best dog Simulation: Dog's Life
Best husbandry Simulation: Harvest Moon: Friends of Mineral Town / Viva Pinata
Best single-player FPS: Chronicles of Riddick: Escape from Butcher Bay
Best multi-player FPS: Unreal Championship 2: The Liandri Conflict
Best co-op TPS: Raze's Hell / Gears of War
Best action/adventure: Okami / Conker's Bad Fur Day / Legend of Zelda: Wind Waker\
Best 2D platformer: Super Mario Bros. Advance 4 / Sonic 3 & Knuckles
Best 3D platformer: Super Mario 64 / Lego Star Wars II: The Original Trilogy
Best fighting game: Evil Zone / Super Smash Bros
Best card-based action: Phantom Dust
Best trivia: South Park: Chef's Love Shack
Best music/rhythm: Um Jammer Lammy
Best boxing: Wii Sports / 4D Boxing
Best squad-based strategy: Overlord
Best graffiti: Jet Set Radio Future / Marc Echo's Getting Up
Best space-combat: Freelancer / Star Fox 64

Honorable Mentions for RPG: Anachronox, Jade Empire, Fable, Star Wars: Knights of the Old Republic 2, Star Wars: Knights of the Republic 1, and Paper Mario: 1000 Year Old Door

Honorable Mentions for FPS: TimeSplitters 3: Future Perfect, Blood, Generations Arena, Tron 2.0, Alien Vs Predator 2, and Turok: Rage Wars

As for the worst game I've ever played, that's quite easy: ET for the Atari 2600.  The game was an unfinished piece of crap with frustrating gameplay and little to no direction.

Best real-time RPG: Kingdom Hearts I
Best turn-based RPG: Final Fantasy X / Earthbound
Best single-player FPS: DooM / Half-Life 1
Best multi-player FPS: Wolfenstein ET / Quake III / Natural Selection
Best action/adventure: Metal Gear Solid 2
Best 2D platformer: Sonic 3 & Knuckles / Megaman Zero 1-4
Best space-combat: Wing Commander
Best Kinetic Novel: Planetarian / Narcissu

I'm still playing Final Fantasy XII. So far, it's been an amazing experience. I've also been "playing" kinetic novels, which can provide great stories and, sometimes, great voice-acting.

BTW Kain: Lvl. 100 Sora in KH1 with Ultima Weapon ;D

*nervously raises hand

I love and despise threads like this in equal measure, so in the interests of fairness I'll go system-by-system

Atari 2600
Best: Solaris. One of the later games for the system and one of the more successful attempts at 3D. It's basically Wing Commander, but more arcadey. Seriously, fire up an emulator and give it a bang.
Other good ones: California Games, Pole Position, Jungle Hunt, Vanguard, Super Cobra.
Worst: Human Cannonball. It's as pointless as it sounds.

Mega Drive / Genesis: Gunstar Heroes. The freshest approach to platform games I had come across in a long time. Was kind of at the forefront of the SNES backlash, hence all the scaling, rotation and deformation effects.
Other good ones: Flashback, Virtua Racing, Jungle Strike, Lawnmower Man, Streets Of Rage 1 & 2.
Worst: Super Thunder Blade. The game demanded more than any 16bit system could cope with. Not to mention that if you sat at the top corner of the screen you could get through almost the entire game without getting shot down. Just play Jungle Strike.
G-Loc is down there too. You can get through the entire first mission using one button (basically when the guy says "Fire!").

Mega CD / Sega CD
Best: Thunderhawk. Glorified shooter disguised as a helicopter flight-sim. Excellent.
Other good ones: Sonic CD, Silpheed (currently getting the X360 tretment I hear).
Worst: Road Avenger, Cobra Command, Night Trap and all those other totally uninteractive "interactive movies".

32X
Best: Virtua Fighter. Bit blocky on the ol' Magic Mushroom (as we called it) but the game was all there.
Other good ones: Virtua Racing Deluxe, Afterburner (arcade-perfect!).
Worst: Doom. Although I enjoyed it at the time, it wasn't until I got a PS1 that I discovered how half-arsed a port it was.

Super NES
Best: Starfox / Starwing. OK, "lamb-to-the-slaughter" time. I loved this game and still fire it on every now and then. Never managed to access the Black Hole but I found the other secret level OK, though.
Other good ones: Mario Kart, F-Zero, Super R-Type, Zombies Ate My Neighbours.
Worst: Plok. Sonic without the speed. Or the good bits.

Playstation
Best: Gran Turismo 1. Mere words cannot convey how class this game is. Still the best IMO.
Other good ones: Tony Hawk 2, FF7, F1 97, Tekken 3, Colin McRae 2.0, Doom, Final Doom, GTA 1, 2 & London, Tomb Raider 1, 2 & 3, Metal Gear Solid, Resi 2, Die Hard Trilogy.
Worst: Rise Of The Robots. Damn, I've played some turkeys in my time but, Christ ...

Playstation 2
Best: GTA San Andreas. No other sandbox game comes close, including the next-gen attempts we've had so far.
Other good ones: The other GTA's (obviously), Quake 3, GT4, Burnout 2, R-Type Final, Black.
Worst: Enter The Matrix. After much deliberation, this one gets the prize for shittiest PS2 game ever. It's more of a rolling demo than a game, showcasing all the bits the Wachowskis couldn't be arsed putting in the film.

PC
Best: Ultimate Doom. I'm with Pho on this one, and why the hell not, eh? It rules, you all know that!
Other good ones: Too many to mention here *cough* Lemmings *cough*
Worst: Juiced. Gimp My Ride gone hopelessly wrong.

Best Mods: Generations Arena, Painkeep Arena, Q2Gen, Classic Doom 3, Shambler's Castle, Titan (one of the finest Doom 2 wads I've ever played), Aliens TC by Justin Fisher (jDoom version).

There are way too many games to think about.. and most of them fit in an area of 'mediocre'. I'll do my best tho, and list the system the games are for.

Best
Pac-Man (Arcade): I've been a Pac-fanatic since I was two.. and I still get in my modes where I just HAVE to play Pac-Man. It's a classic.

Mega Man [classic series] (NES): I've also been a Mega Man fanatic since first playing Mega Man 2. Unfortunately, the classic series kinda died off for me when it made the jump to the SNES. The X series was much better there.

Doom 2 (PC): I love Doom 2. My favorite monsters from the old Doom games came from Doom 2 (Arachnatron and Mancubus). I'll give a shoutout to Quake tho, because without it I'd have never played a FPS game without cheat codes. I also liked Quake's multiplayer better.. but I sucked at it stand-alone.

Team Fortress [Mod for Quake] (PC): This, to me and my bias opinions, is the mod of all mods. Almost every FPS game today has classes and/or a TF(-ish) mod that eventually comes out for it. The original is still the best tho. HWGuy.. FOR.. LIFE!!!

Baldur's Gate 2 (PC): As far as RPGs go, this is the best to me. I loved this game so much, and it's actually one of the very few RPGs for the PC that I've completed (Note: I'm not counting the expansion). That and I loved D&D 2nd Edition. I only ever played this like twice (less than the others on the list), but it's still awesome and memorable. Take THAT, turnip hatin' scum!

Super Mario Bros. [1 & 3] (NES): Kinda like Mega Man, I have to play these every once in a while. I really dunno why. I also can't play the updated versions because of my nitpicks with them, so I have to play the original NES games.. which is probably for the best anyway.

Re-Volt (PC): I'm not much for racing games, but this one was fun. It's like the spirit of Mario Kart, but with small RC racers instead. The only real flaw I can think of is that you have to use faster cars.. even if you prefer a slower one (due to style or whatever). No.. really.. you have to eventually.

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Worst

Pac-Man (Atari): Yeah... this game sucked so bad that it actually hurt. The sound effects were the only memorable part of the game, and only because they sucked so bad.

Ghostbusters (NES): I really have no idea what was up with this game. I mean, it's bad enough that you have to buy your equipment and such.. but then you have to buy gas and avoid wtf-lame swerving drivers on the road to get to where you're going. They swerve across all the lanes and when they hit you, you have to pay for it. ARRRRGH! And don't even get me started about the part with the 22 flights of stairs where you have to rapidly push the A button (or was it B? I don't remember) while trying to avoid ghosts. %!@*&$* impossible!

Uhhhhh... I'm sure I've played more crappy games than just those two, but I really can't remember them.. which is probably for the best.

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As for Gen.. I'm hoping it makes the best list eventually. But as of right now, having only played .99f.. it feels imcomplete, which it is really. I can't put an incomplete mod on it. I just can't.

He's the angriest gamer you've ever heard...

He's the Angry Videogame Nerd.*

* Rated F for !@#$balls.

Edit:  What I find most humorous about that video is that I still have a lot of those Ghostbuster toys.  I completely agree that the glow-in-the-dark ghost trap and the PKE meter are pieces of crap.

Oh hell, we used to have one of those games... thank god they didn't actually make all four of them...

would've been better if you didn't have to have the booklet that came with the games to win

After watching the angry video game nerd I got nostalgic for the old NES, after cleaning/fixing a couple of flip tops... and man do i have some shitty assed games for it that i've almost completely forgotten about,

Gilligan's Island - utter shite little more than walking from one retard who doesn't move for shit to another just to play fetch and carry for some idiotic stuff, and did I mention that you play as the skipper? yeah and gilligan follows you even if it means he falls into a sporking hole his dumb ass should see, or if you're just a little too far from him when you go into a different area.

Teenage mutant ninja turtles - The ending sucks balls, I cheated my ass off with a game genie to be able to get through that *&^%ing dam level and as packed as the last level is I don't see how anyone could beat the game otherwise, anyway in the ending splinter turns into a human saying that it's his true form now I didn't really get into the comic but that pretty much made no sporking sense also -get this- NO CREDITS at the end I guess whoever was involved in this abomination before the gaming gods were so ashamed of themselves that they didn't want anyone to know they made it, rather make everyone think this putrid pile of FAIL congealed in some computer and made itself.

Rocket Ranger - A game that tries to be more like a movie, and FAILS at both the text screens that show up whenever you try to do anything are just too damn informative, you get this wall of story and only a slight clue which of  the 3-4 "action  scenes" this boring crap fest has. Really that's the best word for this game BORING, you send agents around the world and read their reports, boring. you fly around shooting planes, BORING. You get into a fist fight with some army guy, REALLY sporkING BORING. I guess if they thought if the game was set in this 1930's esque world then it should be as boring as whatever home entertainment was available in the 1930's...

Final Fantasy & Legacy of the Wizard & Metroid - Okay these games aren't bad but definitely needed some improvement in one way and that is to tell me where the spork am I supposed to go, god damn how the hell are you supposed to get the best ending in metroid if you waste hours trying to figure out what to do next?

Impossible mission - Probably would be shitloads better if I didn't need an instruction manual to know what the hell is going on or what the computer icons are for, loses points for no music but gains more for that wonderful scream when the dude falls off the screen.

Spelunker - Too hard, it's like... trying to tattoo a guy's flaccid genitalia while off roading on an ATV, it's just that frigging hard, you touch a rock, DEAD. you fall even the smallest distance, DEAD. You stand on the same screen as an exploding bomb DEAD. hell you breath on your controller and you die. I don't mind a challenge but this shit is overkill in difficulty.

In other words if you see these games for the love of all that you care about DON'T PLAY THEM! but chances are if you've read this you're off to romnation or some other emulation site to see if I'm exaggerating.

I'd talk about the wonderful gems on NES but I think I'll stop before I get that WALL OF TEXT thing that some people get here.

Okay, got to rearrange things and now my good ol' Genesis is hooked up as well, even though I don't have as many games for it as NES. Either way time for some of the good  games

BattleToads - A cult classic for good reason, it DEFINES hard in a way that makes ron jeremy jealous sure the second level lets you accumulate an absurd amount of lives but pretty much any level after it is more than capable of taking them away, most non-boss enemies can easily take a third of your life if you let them, and the bosses can just outright kill you without so much as a second to let you think but, as hard as it is simply being able to beat a level gives you a feeling of accomplishment.

Zelda, the first one - The only NES zelda I liked, it may be hard at times but it... it's freaking zelda man! how could you not like it?

Gyromite - I may be the only person who likes this game but hey, I think it's fun and quirky, playing it by yourself and without the ROB thing can be a bitch but that is the only way I've played it so... yeah...

Battletoads/Double Dragon - Two great tastes that don't taste like ass together, seriously it has more beatings than normal Battletoads but less repetition than Double Dragon, I'd almost say it's the perfect beat em' up.

Gilligan's Island - On second thought this game isn't too bad, when you put it in a microwave.

Earthworm Jim - It's fun, has all the sounds you'd expect from a cartoon and the humor in it is just too funny, Such as hell having the song Night on bald mountain play only to be interrupted by elevator music, complete with the sounds of screaming.

Sonic 3 & Knuckles - If god himself didn't do anything to help make this game I doubt that he could make it any better, it's just that good.

Star tropics - A bit of a modern timed zelda, you're some guy looking for his uncle in a bunch of tropical islands, and you're armed only with a... (dramatic pause) frikkin yo-yo... has some humor but some parts kinda really piss me off, every time you get to a new village you have to talk to everyone before you get to go into any dungeons, but still pretty fun I think.

Wow... I can be short and succinct when I'm being nice... either way I get my SNES back on the 19th, I'll talk about those games a bit after then.