Geez so many, I'm strange as in I prefer RTS games over everything else.
Homeworld 2 (Complex mod), Company of Heroes (Blitzkrieg mod), used to play Dawn of War years ago every day. Loved the World in Conflict singleplayer. Empire at War was too... small? Selection of units was too small (which is why I added all those mods to my other RTS games). Rise of Nations is always good for LAN's which is the only time I play it, same as Age of Empires 1 and 2 (3 can go die). Played Battle for Middle Earth 2 for a little but that got boring as well (the too little selection of units thing).
If any of you play Homeworld 2 or Company of Heroes I implore you to download the mods I mentioned, adds sooo many units to the game and also changes a lot of damage profiles and tactics etc, so much better than the Vanillas.
EDIT: Oh yeah, play Total Annihilation when I'm feeling like it, still a great game only barely showing it's age, as well as Supreme Commander: Forged Alliance.