Half-Life 2.
Even if half of the stuff we have seen in the videos is emergent and not scripted, it's going to be wonderful. Heck, if it was all scripted, it's going to be wonderful.
And for community support, HL2 has so much to build upon (and DOD2 and CS2) that it's unbeatable.
Speculating on, I'll say UT2K4 will also live on until UT2K5
... it's great and already Alien Swarm and the Community Map Packs have shown great potential for expansion.
Riddick, Doom3, Stalker, Halo 2 - all will be good, but will not be remembered as classics (as HL1, Halo, Doom1 are)