Medieval Total War 2 + expansions
If you don't mind old games though...
Also, if you don't mind fantasy
-Warcraft 2 + expansion
-Warcraft 3 + expansion
-Lord of the ring Battle for the Middle Earth 2 (LotR BME2) + expansion
If you don't mind a bit older than Medieval era
-Rome Total War + expansion
Magic with those games is that they are very easy to find, very nice game play (Medieval Total War is OMGWTF realistic in both graphics and game play).
Oh, and if you're comp is pretty decent and you don't mind playing with muskets instead of swords
-Empire Total War + expansions
You can see I'm a fan of the Total War series... well wait till you see how the game play is complete and graphics are amazing. AI is also very complex and performant. At one point, if you're troops have been fighting for long enough (they gain veteran pts, so you,ll know) they'll start helping each other on the battlefield (like 1 block the other strikes).
Also, all those games (beside Empire) are cheap. You can get em for about $20 each (got Medieval + 2 expansions for $20 + taxes in a store...)