Starcraft is good, if SC2 had LAN it would be the RTS of choice.
Dawn of War, Company of Heroes, Age of Empires 2 (AoE3 is ok but AoE2 is the better game and also will run on ANY computer). Warcraft 3 is great too. A lot of the Command and Conquer games are good too. Just not CnC4. Don't go there.
Total Annihilation is the precursor to SupCom and is a fun little hectic game, definitely worth a few fun games.
That's about all I can think of off the top of my head, I think I covered all of the best ones.
EDIT: Hah! Ninja'd on almost everything. I should read more before I post.