Hence the title being KZ2
single player I have to go with Killzone 3, if it would have had the serious tone to it like Killzone 2, it would have been epic but they mad it more superheroish in tone which I didn't care for. But the gunplay & fun factor of the game play in Killzone 3 outdid Killzone 2 in singleplayer
now when you get to
online?

there is nothing KZ3 can do with Killzone 2. Killzone 2 online gunplay & visceral feel of war is unmatched by anything this console generation. I really feel the game was ahead of its time in this regard & most simply couldn't handle that real 'war feeling'.
In Killzone 2 when you popped an opponent it really felt like you just dropped someone...and the kickback you'd get when you squeezed on off (the rumble from your controller just added to that affect.) The 23 ways a bullet could travel really made that real as well. It made you focus with your shot & be patience...once rewarded with that chirp sound.
I think the thing that made that feeling of focus so rewarding was that it would be complete chaos going on around you. The graphics, dust particles, shrapnel, Bodies flying, explosions rocking your aim...but the game demanded you ignore all of that & focus your aim, lock on your target & get the objective done.
It was a truly beautiful experience...one of my most fun times from this generation.