I don't really play mp much but at level 37 i think.
SMG's: Vector, it has practically no recoil, a pretty good rate of fire, and I find it pretty good with a suppressor.
AR's: M27, once again, low recoil, and it goes pretty well with target finder/red dot sight
Shotguns: Remington 870, kinda gimmicky, I only use it on Hijacked, but is strong overall, especially with long barrel
Snipers: DSR 50, once again kinda gimmicky, but I use this on a few more maps than the shotgun.
LMG's: Even though I did really well with LMG's in MW2 and MW3, I just don't really care for using them in BO2
Then the B23R, just a plain beast, it has saved me a bunch of times, now if only it had a mag larger than 15 (21 with extended mags).
I haven't unlocked everything yet, but I feel I will like the M8A1 and AN-94 a bunch, especially M8A1 with 4 round burst. Currently remembering beasting with the FAMAS getting 30 kills in a game in TDM on MW2