I don't think I can choose a favourite between Halo and last gen's Rainbow Six 3.

Honourable Mentions:
Half Life series
Doom (including 3)
Duke Nukem 3D
Killzone 2
Battlefield 2
Unreal
Borderlands
Wolfenstein (excluding the horrid iteration released last year)

...What can I say? I love me some classic FPS action.

How did I forget CS and Ghost Recon?  Add those to the honourable mentions.

Half Life series. Still the king for me for SP campaigns in FPS.

Unreal Tournament remains the best MP experience for me in terms of gameplay types and excellence at the time.

call of duty and battlefield bad company. If there was a call of duty game that had all the features of battlefield the world would be one.

Halo
HalfLife
DukeNukem
BattlefieldBadCompany

and of course
FPS by idsoftware

Halo and Battlefield.

The lack of Bioshock is disturbing.

For me personally Bioshock was a solid game overall in its presentation and story-telling but the FPS mechanics and gameplay itself was kind of a let down. It felt kind of weak to me after having played Condemned 1 prior where similar mechanics (gun and melee combat) were used but felt much more compelling and painful.

And I also forgot to add F.E.A.R. to my list. I don't think I've felt more satisfied shooting a gun in a game as I have with the pistol in FEAR.

Haven´t played Metroid yet so I am going with Half Life.

Socom b4 Slant6 came up with the mess known as confrontation.

MAG is excellent though.

Whats so special about Bioshock guys?I still havent played it.