Eh, Killzone 2 deserves the award in my book. I played that thing non-stop for months after it came out. The dynamic rounds, class system, and clan system kept me hooked, and I really like the heavy feel of the controls.
Left4Dead 2 is the only shooter I feel comes close this year, because the coop is just so damn fun. I've put over 20 hours into it since I got it, which is a lot for a time constrained guy like me. Honestly, I'd say these two are my favorites FPSes this gen - L4D2 for the coop and Killzone for the singleplayer and competitive multiplayer. Plus I love all the custom campaigns you can find for L4D on Steam. :D