I mostly play core TDM and for me it's the R-870. I'm pushing 25K kills with it and it just works for me. Second I'd have to go with the Skorpion.
Like others have stated, there are a lot of variables. Game mode, style of play, sensitivity settings, playing alone or as a team, etc that in my opinion can change the productivity of any given weapon.
Favorite class:
R-870 (laser, long barrel)
Perk 1 (greed) (light weight, ghost)
Perk 2 (scavenger)
Perk 3 (engineer)
C4
EMP grenade