I have a question: does "unlimited Ammo" make the clip size infinite (i.e. no reload) or is there a giant pile of ammo clips next to me? Do the weapons still overheat if they have unlimited ammo?
If I had to choose, I'd probably go with a Bolter with Hellfire, Inferno, and/or Metal Storm rounds. Any of them will do, although Inferno bolts would be sadistically satisfying as they would immolate the target from inside. >
If a bipod is allowed (it's still in my hands but resting on the ground) then I'd take a Heavy Bolter with Standard and Hellfire rounds. Standard rounds for long shots, when they get close, switch to hellfire and let the needles & acid do their thing. .100 mini-rockets fired from an MG is just too awesome to pass up.
Although if I never had to recharge it, the Electricity gun from Killzone 2 could be worth a few lulz, as it would never shock you the user. Seeing monsters shake all over the place from the lightning bolts would be pretty cool.
If I had to choose real guns, I'd go with an XM214 mingun and incendiary rounds. Aww yeah, it's the minigun from Predator, with bullets
on fire*. No need to aim, just point and annihilate.
*you know what I mean.