Medieval II : Total War, defiantely. the more I learn about this game, the more excited I get. I'm having so much fun replaying RTW with different factions or simply using different strategies. the game is so deep and has so much to do, MTW2 has so much potential. All the enhancments made to the gameplay will help make this game one of the best RTS ever made, mark my words.
I haven't followed Supreme Commander very closely, but from what I've seen it looks very solid and very promising