The MP5 was my favorite gun in COD4, but in BO, it moves around way too much and is absolutely horrible at long range. The AK-74 is better, doesnt move around as much, and you can actually hit something at long range if they are not moving.
As for Assault rifles, that G11 is a waste of time, dont bother. Sure, it has some power to it, but it blocks your entire view of your right side becuz its so massive. You only get a choice of 1 scope for it and it too sucks.
I like the FAMAS, decent power, great rate of fire and is crazy accurate. Its almost too accurate at times, if you hit L1 (on PS3) and its just slightly off the target, you'll never hit the damn thing. The AUG is a good alternative to this if you want something with a little more margin for error. I mean this by, if you are just slightly off when you ADS, it has just enough movement to find the center of the target to get the kill, but not too much movement that you'll be all over the place.
Sniping has been ruined in this game. Sure its possible to do it, but its not as fun anymore. I'll stick with the AK74, FAMAS and AUG and Im good to go. Oh, and my trusty Crossbow and Tomahawk