Depends on the game/map.
I've always loved SMGs, but in Black Ops they all just suck, except for the 74u which I refuse to use.
Assault rifles as the most effective, but they're boring to use.
Shotguns are cool, but there's only 1 or 2 that are worth using, (Stakeout and MAYBE spas 12)
Sniping is awesome, but it's just not practical. All the maps have so much CQC, and the snipers have been nerfed a whole lot.
LMGs are fun, but the slow movement speed annoys me. I'm a really aggressive player, and it's hard to be aggressive with an LMG.
So usually I use snipers on more open maps, but not the long range ones like Array. More like Havana.
I use Assault rifles on the big maps, or things like Villa with lots of long passageways.
SMGS/Shotguns I use on close quarters maps like Launch, Nuketown, or even Hanoi.
LMGs I use on medium sized maps like Radiation.
However, I'd like to point out that I really enjoy doing pistol only games, as they require more skill than shotguns/smgs, and you have to play like you have one. It's really fun, and I love it.