for me, its the DSR-50 sniper rifle. if you can take one of those and do as well as someone with a submachinegun with rapidfire, then you can do well with any other gun in the game. got diamond camo for all my snipers ^^
The best guns are Assault Rifles, Shotguns, Light Machine Guns and Sub Machine Guns. All guns within the game have there own gain of experience when put into use and can get individual Prestige levels.