I can only recommend some old games, which are mostly out of stock. If you don't mind *cough*pirating*cough*. Also, I think I don't need to mention big names like Starcraft or AoE.
Z - ( The name's pretty hard to google for; try googling Z+Bitmap+Brothers). The game involves no base building or resource management. You have to capture factories/hardwares scattered around the map. My all-time favorite.
KKnD - If you like C&C, you'd probably also like it; it's very similar. I can almost say it's a clone.
The Settlers - The games in the series focus more on city building. I've played only The Settlers 3. It requires some serious micromanagement.
Homeworld - Space RTS with unique 3D gameplay, and a compelling story, which made me cry. :,(
Stronghold - Medieval RTS that focuses on siege warfare. You build a stronghold and fend off enemy's soldiers and siege engines. The first Stronghold is nice. I've never played the sequels but I heard they weren't as good.
Rise of Nations - Pretty much Civilization in the form of RTS, and CIV is an epic game. ; )
I've never played Supreme Commander, Warhammer series, or Total War series, but they might be worth a try.
Since 2000s onward I've stopped paying attention to mainstream and turned to indie instead, so I don't know much about today's mainstream RTS. Actually I haven't played many indie RTS games either. For a good indie tactical RTS, try Atom Zombie Smasher. And If you also consider tower defense as RTS then try Plants vs Zombies or Revenge of the Titans.
