Hmm..
I've always enjoyed the Command and Conquer series of RTS games, since they're all fast paced, and best of all there's barely any unit restrictions XD NO MORE 'YOU CAN ONLY BUILD 20 UNITS THEN NO MORE!'
Then there's also the Dawn of War series... While they do have restrictions the storyline is great, and it's still quite action packed.
And Universe at War. Those are pretty much the three series (except UAW, it's a single game) of RTS that I play.