The ISA M82g seems to be the favorite (the one with green light) for most people. It's great for ranged encounters, while still being decent in close quarters.
My favorite will always be the StA-52 LAR (standard Helghast rifle), just like it was in the first Killzone. It's a bit less accurate than the M82g, and the bullets are of a smaller caliber, but it has a much faster fire rate. It's great in close encounters, though I get owned in ranged matches (particularly 1v1 matches against outlaw :P).