This is pretty easy. Stainless Steel for Total War: Medieval 2. It's on something like it's sixth or seventh iteration now, and it's expanded on the base game so much I barely even remember what the base game was any more. Whenever I speak fondly of Medieval 2, I'm thinking of Medieval 2 by way of Stainless Steel. It's an absolutely massive overhaul of the entire game, changing the map, the factions, the units, the AI...if something can be changed/improved upon, right down to base systems, it has been. It's the closest thing to Medieval 3 we have. It makes the game a gloriously rich and meaty experience. Newer Total War games absolutely pale in comparison.
http://www.twcenter.net/forums/showthread.php?s=0c5fa3ee8a4b8593409cf2c3faa6a32b&t=417435