I think I'd want to play Halo 2 the most, because I had more fun with Halo than I did with Half Life or any prior Doom.
Half-Life 2 will be fantastic, no doubt, I just don't see myself getting into it. I've kind of dropped PC gaming over the past year and a half, since I have no desire to upgrade my PC to be capable of playing the newest and greatest.
Doom 3 looks like a great engine. Note, I say "engine", and not "game", because the things that I have seen from Doom 3 so far, such as spiders with upside-down skulls on their backs, just looks retarted.