Personally I preferred Killzone 2. Killzone 3 was utter shit in my opinion.
how so ?
I thought it was fine :/
Think of it this way (if you can); Battlefield: Bad Company 2 was my favourite Battlefield game (along with 1942, BF2 and 2142) even though Battlefield 3 is newer and better in many aspects. Battlefield: Bad Company 2 was the only FPS in a long time where I had a lot of fun on it and Killzone 2 shares the same qualities. KZ2 may have not been better than KZ3 but I loved it compared to KZ3 (if that mad any sense). >.<
It's like, for another example, some people say Call of Duty 4: Modern Warfare was the best even though there are newer and better ones of the Modern Warfare series.