I really liked the Doom Series, I really loved Duke 3d, but I think the game that really got me the most into gaming is Quake & Quake2, followed very very closely by Half life. Then, my third favorite game ever is Transport Tycoon Deluxe.
CounterStrike is just a mod for half life, IMO BF1942 Desert Combat is a much better game. I liked HL: TFC the best, however, RTCW: ET has replaced it as my team game of choice (well, and BF1942 & Vietnam)
Starcraft was OK, I liked Total Annihilation quite a bit more (with Core Contingency mod). I also like the age of empires series better.
Then theres Carmageddon and Carmageddon 2 (I didn't care for 3). IMO these were much cooler than the GTA series ... (Though I have killed many an hour playing langames of GTA and GTA2).
Unreal is kinda bland IMO ...
What about Farcry? I agree its not the greatest game ever, but cmon' Halo? How can that possibly compete against Far Cry? (OK maybe apples and oranges here ... but Farcry is just soooo much better)