Depends on you man. Every weapon has its own special ability. The SWAT-556 is perfect for watching over objectives. The PDW and MSMC are more of a aggressive play style. The AN-94 is more of a pushing objective gun. The FAL OSW and SMR are high damaged weapons.
Personally my favorite guns are: AN-94,MSMC,PDW,SWAT-556,M27and Ballista. Black ops2 has balanced guns meaning that there are no dominate guns that everyone uses. I find different weapons on ground rather in mw3 you would find tons of ACR and Mp7.