I recently played KZ2 and 3 for the first time last month and I loved them. SF is good but the singleplayer just isn't....fun? This is weird to say, but there is a lot of time where you aren't really doing anything fun, and it usually happens when you aren't shooting stuff, I'm sorry to say. There are like two whole chapters where I felt like I was playing Killzone: Myst. Except the puzzles/exploration isn't very interesting. And I'm not even like a "oh I need to be shooting stuff to have fun" guy either. It's strange, and disappointing because there are a few really good, open ended, shooting sequences. I enjoyed KZ Mercenary a lot more.
Multiplayer is pretty fun though, but there's a pretty small population.
AC4 on the other hand... i think I just put in my 60 or 70th hour on that bad boy. Great game.
And NFS Rivals is damn good too.