Age of Empires 3 is currently my RTS of choice. Excellent, excellent game - if you are into multiplayer then it's a very econ driven, custom card specialising, statistical nightmare but it's really, really, really good fun when you are up to speed. Plus it just looks so delicious.
Battle for Middle-Earth 2 is also very good indeed. This is by the same team that did the Command & Conquer series and they have gone back to form after some 'innovative' changes they tried out for bfme1.
Rise of Nations should be out soon also, and that should be fairly big.