Honestly? I liked Prey the best for campaign all around. The tweaks were just enough to keep me quite interested, despite its shortcomings. I very much liked the flipped gravity and the out-of-body gimmicks. It's one of the few FPS campaigns I played through multiple times gladly. Portal is obviously brilliant, though I don't think it's really an FPS so much as a puzzle game with an FPS camera. I personally wouldn't count HL2 either as "this gen". It was released in 2004, and I'd say this gen started in fall '05.
Honorable mention for Bioshock, Halo 3 and COD 4/6 for excelling in certain areas, though each had its flaws. (Bioshock - ending and "hallway syndrome" too often, H3 - story, COD 4 - monster closets and cheap deaths, COD 6 - preposterous, ridiculous, ludicrous story).
Didn't play KZ2 campaign, so can't judge that. Resistance campaigns were just ok for me, though #2 had some cool moments. Battlefield campaigns suck, IMO.
Can't we all just get along and play our games in peace?
