I'm probably late to the party on this one, but I've been having a hell of a lot of fun with the
Stainless Steel mod for Medieval II. Some very cool changes in this mod. You need to worry about resupply out in the field, and armies lose or gain morale depending on their previous battles. They've changed the way recruiting works, and made it so that mercenaries are really integral to the game in that they cost less than your own troops, but are double the amount to maintain. Basically, your indigenous manpower will often be limited, so you'll have to rely on mercs a lot more, at least for short campaigns. In the long run, they'll ruin you. Cavalry is way more important in this mod, reflecting the actual importance of knights historically. I know I used to only bring a couple of cavalry units to chase down runners, but they're really a decisive arm this time around.
They've also added new factions, shitloads of new units, extended the map out to include the Persians, and even given generals more speeches with great voice actors and lots of profanity. This is probably the most thoughtful mod I've ever seen put together from a historical viewpoint. These guys really did their homework. To be fair, if you weren't down with the micromanagement in the vanilla game this mod will really drive you batshit. If, on the other hand, you tackle micromanagement with the kind of OCD-ish tendency that I do you'll love it.