I like the ones that kill people quickly. Just when I think the AK-47 is the best, I get lots of kills with the M5 or MP5, whatever it is. And you can really take someone out with one of those single shot guns, or that 3-shot burst gun. But I don't aim well enough to use them often.
I do like the guns with silencers, it makes them sound cool.
The AK-47 with a silencer is the one I keep going back to.