Europa Barbarorum for Rome: Total War is probably my favourite. It's hard as hell, but all the rich detail makes it worth it. They really put every ounce of the engine to its limits. It's chock block full of historical info, really good unit and faction design, the map is enlarged, there is a solid attempt at adding a political system and the areas of recruitment are awesome. Now you can actually play Carthage with an army of mixed Libyan, Numidian, Iberian and Gallic mercenaries, for example. Actually, the mercenary situation has been greatly expanded upon in the mod, which makes it very interesting. You can wind up with all kinds of interesting units in various parts of the world they wouldn't normally end up.