Starcraft is, IMHO, the best RTS ever.
As for FPS game, I've been playing HL2 and CS:S for a couple months now and I'm not gonna lie, they are both awesome games. But neither of them are the kind of game that keeps me drawn in for hours. The only FPS game that has done that is Rainbow Six 3: Raven Shield. I've been playing RvS since the demo came out (almost 2 years) and there is something about it that I just love. Whether it's the realism, the gameplay, or whatever, I just can't get enough of it.
Just my $.02