Not Half-life, because it's too easy to get lost, the puzzles are frustrating, and it's not that satisfying shooting the weapons in that game.
Not Doom. Lacks storyline / emotions.
Not Halo. No mouse. Horrible vehicle controls. Too long.
Not Quake / Unreal tournament. (Though you should add them to the poll). Because they feel more like tech demonstrations of their respective graphics engines rather than full out games.
Not Medal of Honour. (Again, need to add it to poll). Because it's "too gamey".
So, Call of duty, by process of Elimination.
*edit: I would say Portal, but that's not really a FPS, more of a puzzle game. BTW, I think Valve should ditch guns all together, and add more content to it, that would make the Half life series better.