For me, it would be the SVU. It's scary good...I'm actually surprised they haven't touched it lol.
That SVU is nasty. It's an absolute beast from mid-long range. I've just started using the M14, and that's a beast as well if you can get headshots. I've gotten some 1SK headshots with muzzle break at ridiculously long ranges. I like all the marksman rifles. I can't wait to delve into the IA2. it seems to have a similar profile to the SMR (my favorite and top used BO2 weapon)
My top used weapon in this game so far is the Bizon. I guess my favorite weapon is that or the SVU, depending on the map.