Half-Life edges out Portal for me. The Half-Life series does everything I love about games, and I really do like the idea of episodic gaming (though they gotta do something about the wait between entries). Episode 1 was great; Episode 2 has been my most-anticipated game since I got through 1.
That said, Portal seems wicked, and all the little tidbits I've heard about it beyond the basic concept -- that it does take place in the Half-Life universe, that that voice actually has character development -- really has me curious. And the basic portal concept itself is of course great -- anybody else played
Narbacular Drop? It's a short little freebee using the same portal concept with a much older 3d technology. It's what the folks Valve brought aboard to make Portal did to earn that hire.
As for TF2 ... not really interested. Multiplayer fragging isn't my game, whether I be pwning the n00bs or the n00b being pwned. Multiplayer co-op appeals to me, so Left 4 Dead, as of midnight tonight when Steam unlocks my pre-loads, becomes my most anticipated game.
Can you add a secondary poll? Find out what system everybody's picking it up for (me: PC).