I've played Killzone, Ghosts, Battlefield, Resogun, and Contrast thus far.
My favorite has been Battlefield 4. It's like 3, but more stuff, new maps, and better balance and graphics.
I've only reached the trains in Killzone, but surprisingly, I'm not quite feeling it. The game is gorgeous, but as a fan of 2 and 3 (I even kind of liked one by the end of the game), the actual gameplay doesn't seem to work well. There are a lot of good ideas here, but I don't know that any of them play to Guerilla Game's strengths. They had the tight warfare with slow but satisfying progression through a single battlefield thing down with KZ2. Absolutely incredible. 3 was less so, but still serviceable. Shadowfall can be so spacious, the encounters always feel a little rough and awkward. Also, I miss the old cover system, and I don't like that you can't swap out the boring main rifle.
Wait, this wasn't a lightly complain about Killzone Shadow Fall thread? Whoops!
Anyway, BF is great Multiplayer wise (though dying a lot because I don't know the maps kind of hurts). Singleplayer probably isn't amazing, but it seems like a step up from 3 from what I played.