If you are primarily interested in an improved AI, then you should look either at MarvinKosh's Space Weapons Fix Mod, Tolmekian's TechTree Fix, or my own mod, Autumn Twilight.
MarvinKosh made some very big changes to the tech trees, planetary improvements, and other parts of the game. Mainly to optimise everything for the AI. It plays quite differently from the vanilla game. When I tried it a little over a year ago, the AI handled itself quite well, but not all of the races had been done by that point. I don't know how much has changed since then.
Tolmekian's mod is closer to the vanilla game. The most obvious change is the removal of the racial weapons (like Seeker Missiles) and the ultimate weapons (like Doom Rays). A couple new techs and improvements got added, and several existing ones got improved. From play-testing the mod, I can say that the AI does a good job. Not perfect (when is it ever?), but definitely better than in vanilla.
My own mod rebalances the game, with the main-emphasis on keeping everything lore- and AI-friendly. The changes range from minor (like re-adding the weapons, that got removed in TotA) to major (like a total overhaul of all weapons and defenses). However, it still is very similar to the vanilla game. Currently, I would consider the AI to be decent, but I'm the last person you should ask to rate my own work. My opinion of myself isn't the best, to say the least.
Well, I hope this overview was helpful.
