AK-47 is by far my favorite hands down, as it's accurate enough and one or two rounds usually drop a target. With the M16 variants I've put multiple rounds in guys and they just keep right on truckin' like it's airsoft or something. It's maddening.
I haven't used the AK-74 much, but it seems to be stunningly effective when I happen to pick one up. The jury is still out on that one, and I guess the same goes for the scopeless M4 or M14.. whatever it is.
I almost like the scoped version of the M4, but I tend to use iron sites a lot when I move and the scope gets me killed because I move like I'm drunk. It also seems to do garbage for damage.