+1 for Sins of a Solar Empire Get the Entrenchment add-on and patch it up using the Distant Stars mod - looks stunning (thanks to Bailknight et al) For sheer epicness, nothing comes close, although I found it can get a bit repetitive in single-player. Also, no campaign mode. Having said that, if you do dive in, prepare to burn weeks of your life away.
Personally I'd go for Age of Empires III I don't know why but it always keeps my entertained every time I load it up
I always end up enjoying the older RTS games more than the newer ones. AoE II, Command & Conquer (The original one), C&C: Red Alert 2 Awesome AWESOME games. You can still easily find people to play RA2 online. Worth a go.
Woo! COH! Adding on to the C&C series, Generals is a good game. Just remember to mod it! AOE III is cool aswell.
Not so much SC2, but I'm tempted to go SC:BW /thread On a more serious note: Supreme Commander (1) Company of Heroes World in Conflict (for something slightly different, especially multiplayer with good mates) Age of Empires 2 Star Craft Brood Wars (you know you want to, in spite of the fact that it's older than some very young religions by now)
world in conflict is excellent (a decent story in an rts, amazingly) Sins of a solar empire is different, but awesome. and can't beat a bit of good ol' AoE2 (although settlers 3 comes close, shame the horrible UI ruins it a bit)