Medal Of Honor is a close second. Whilst MW2 has more varied gameplay, I prefer the authenticity of MoH. Plus it just looks freaking sweet.
Beat me to it. Multiplayer Goldeneye was the bees' knees. In fact, I'm still playing it. I just beat the mission where you have to board the train today. It was my first, and probably favorite Fps.
I've got to give a few honorable mentions though. Evanesance got it right with Killzone. I freaking love that game. And I have to admit, I've had quite a few amazing Halo sessions. I can't name names though. I had fun with all 3.