I've been trying out pitting the bots of different Generations against each other in team DM matches, as well as 5-way FFA matches with Sarge, Doom, Ranger, Grunt, and Visor, and come up with this ranking of Generations in the hands of bots, from strongest to weakest:

1. Earth
2. Doom
3. Arena
4. Strogg
5. Slipgate

Earth generally has solid performance with all but one weapon (see below), and the best starter (the MP-40). Sarge seems to have gotten a major power boost, and trounces even Razor in Earth-vs-Earth matches. The sniper rifle is a monster in the hands of a bot. Their major flaw is with the Napalm Cannon, as they're as likely to kill themselves as the enemy with it.

Doom has crappy starters (and the Doom characters are too fond of the chainsaw), but once they get their hands on one of the weapons lying about the map, they're extremely dangerous. The plasma gun and SSG are especially dangerous. Doom comes out on top, being a Tier 6 bot, and none of the bots seem extensively tweaked.

Arena is average, and has no problems with any of their weapons. Not surprising, since the bots were coded with the Arena weapons in mind.

Strogg have a decent starter and a wide assortment of weapons. However, their arsenal is the jack of all trades but master of none, excepting their powerful railgun. Except for the chaingun, none of the Strogg weapons can really put out a lot of damage quickly, which makes things worse. Grunt seems to have gotten a skill boost, and Tankjr feels a bit stronger too.

Slipgate are too specialized. Without the rocket launcher, they're dead. The only other weapon that you can feel comfortable at all with is the super shotgun. The nails fired by the nailgun and super nailgun feel even slower than ever due to the larger architecture, and the two classes you can really hope to hit with an NG or SN (Earth and Strogg) will probably kill you before you can wear down their tank-like armor. But once they have the rocket launcher, they're very dangerous. Again, the iconic character gets a boost, but I was surprised to find that Uriel was always dead last in every match. Uriel was the tournament boss from tier 5, and it doesn't seem right that he should get schooled by some wimpy bot like Mynx.

Very intresting.
I have often found that the slippy twin bird gun is a good weapon. I totally agree with you about the nails as well.  

If you did these tests with players, rather than bots, how would the results stand then? someone in wirehead must know.

All I know (as a constant class-random player), is that the order in which I rack up frags with the most ease:

1. Earth
2. Doom
3. Arena
4. Slipgate
5. Strogg

In Generations .99e.
Note that this is about normal 5-10 player FFA games, taking in account variations because of map/item placement and selection of opponents. So don't bring up maps that have no railgun, or maps that have weapons few and far between etc -- I know those are harder for specific classes. This is just on the whole & guesswork based on my personal experience ;]

Whenever I play Earth, Arena or Strogg, I jump immediately into the fray, knowing that their spawn weapons are useful indeed. When playing Doom and Slipgate, I usually take a detour and pick up a weapon or two.

When it comes to bots, I've known Slippy bots to be quite dangerous with the shaft, too...

I've found that the bots in Gen actually tend to be smarter than in VQ3 anyway, that said, I play Gen on a lot of custom maps not bundled with the mod (they actually go for the MH in Chronic, which doesn't happen in VQ3), but for the most part Arena bots pretty much wipe the floor with all the other classes. Almost every single-player game I play sees an Arena bot come out on top (behind me, of course) and the only other class capable of this is Earth. Slippy-wise, the Zael bot bundled with Castle's Q1DM6 rip:

http://www.planetquake.com/lacedneptune/misc07.html

is particularly deadly with the shaft. Doom and Strogg generally don't give much hassle.

All of the above were tested on Hardcore level.