I haven't played any of them, but I've heard very good things about BKB SuperMod, XL Mod, and Medmod. Also, if you like Age of Empires and more faction-specialized unit rosters you might try the ZX Mod.

As far as a single definitive choice I don't think there is one based on what I've read. You'd probably have to try each of them to find a personal favorite, though you could search the modding forum or some older threads from this one to get some of the pros and cons for each beforehand.

Ajax