Which campaign did you prefer the most, Killzone 2 or 3? 
 
I'm siding with Killzone 3 just because I think that it's more memorable than the second game. I barely remember what went on in Killzone 2's campaign. I only remember the ending of it where your hate for Rico multiplies by 100. Also the diverse environments in 3 were pretty cool.

3 definitely, I enjoyed the set pieces more, and everything in general was improved gameplay wise. The more varied environments helped aswell.

" Which campaign did you prefer the most, Killzone 2 or 3?  I'm siding with Killzone 3 just because I think that it's more memorable than the second game. I barely remember what went on in Killzone 2's campaign. I only remember the ending of it where your hate for Rico multiplies by 100. Also the diverse environments in 3 were pretty cool. "

Don't read if you don't like spoilers! 
 

" @Bucketdeth: I completely agree. If you're judging based on story, Killzone 2 wins, but I think KZ 3 is just superior in terms of content. I enjoyed the campaign quite a lot despite the sad story elements and essentially making most of the Helghast look 100% british space nazis. The developers seem to really not care about their fiction. I didn't like Killzone 2 as much just because most of the areas you're traveling through seem to blend together.  "

I can't effectively put into words how much I hate Killzone 3's single player. But my first thought after seeing the ending was "I hope everyone responsible for this campaign got sacked or gets sacked." 
Killzone 2 had problems of blandness but compared to 3 it is fine art.