I disliked Halo 3.
I liked Killzone 2, the campaign was decent and it was the first online multi-player that got me to play for longer than 30 hours.
I vote Killzone 2.
Had this been Killzone 3 vs Reach, then Reach would have won hands down.
Bet with Conegamer and AussieGecko that the PS3 will have more exclusives in 2011 than the Wii or 360... or something.