Horizon looks great, for sure. But Halo 4, imo, now takes the crown of console graphics king. The last quarter of the campaign is stunning to behold. (and I kinda despise my 360)
Edit: I did just pop in my copy of KZ2 and its still massively impressive, probably need to get KZ3 again and compare them side by side, and there appears to be a whole lot more going on in any given seen in the killzone games, but I'm still leaning towards Halo 4.