... has anyone else ever played the original 2600 Pac-Man?
*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).