UMP from MW2. I remember on Quarry seeing a bunch of dots on my uav inside one of the buildings and just randomly hipfiring into the wall and got a multi-kill. Gun was so overpowered. Killed in two hits and, despite what the stat screen said, that thing had the range of a sniper. Speaking from personal experience here.
Don't know the name of the gun, but it's the second LMG in Black Ops 2. It has a super high fire rate and almost no recoil. I think it hold around a hundred bullets. I honestly feel cheap just mowing dudes down without having to reload. Slap on the scope that puts red squares over enemies, a quick aim grip, and whatever the thing is that increases accuracy and its all over. I set all my perks to whatever helps me get around fastest and I can move like I have an assault rifle. Really, I'm surprised more people haven't discovered this.
The BAR in Call of Duty 3. Good for close-medium range, and can also function as a sniper rifle with single shots. I still play COD3 occasionally with the small group that still plays, and I can usually top the leaderboards with the BAR.
"I just wasted a bullet. Don't waste your life." -Big Boss