After playing on combat training and trying out all the guns and kill streaks, I think the M8A1 with the select fire attachment is the weapon everyone will end up bitching about (i.e. the famas/ak74u of BLOPS).
It's like a G11 on roids.
Most lethal score streak combo: Swarm, Dogs, Warthog
I was able to rack up 50+ kills off these three alone in combat training 1 vs 9.