1) Starcraft (My childhood in a nutshell)
2) Warcraft 3 Reign of Chaos (My older childhood in a nutshell)
3) Dawn of War (My barely teenage years in a nutshell)
4) Star Wars: Galactic Battlegrounds (I actually didn't remember the name of this game until I googled one of the cheats I remembered)
5) Some other one I'm forgetting. 

I also love all of Paradox's Grand Strategy Games, but they aren't RTS, so pft.