Oh, and to add to what Tamerlin said...
Cradle's AI is a killer
Cradle adds another 3000 years to the early game, and a ton of other additions. It is primarily focused in (you guessed it) the ancient period, but is playable to the end.
Medmod, focuses primarily on the medeival, has a ton of additions to the game. I don't find its AI to be as "mean spirited" as it is in Cradle.
World at War, is a WW1 to WW2 era mod and its AI has been known to kick the @@$#$@ out of the human player (happend to me the first couple of times that I tried it
) This is probably one of my favorites.
I've never played Call to Conquest, but I've heard only good things about it.
Super Apolyton Pack, is CTP2 the way it should be (if Activision hadn't rushed out the game) and if you add GoodMod to it, it makes for a very enjoyable experience.
Try them all, you're sure to love them