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.
Every gun short of a shotgun has the range of a sniper. Every gun's bullets will go across maps when aimed properly.
But yeah, that aside, if we're talking about a gun that appeared in multiple games, it would be the AK-47 (and the other variants of it)
If we're talking about a gun that appeared in one game, it would be the MW2 ACR, SCAR, or UMP.
Kar 98 from COD 1,2 and 3. I was BEASTLY with that gun.
Or the MP5 from COD 4.
This right here. The Kar98K was such a great gun! Looked and sounded great, had nice animation and usually dropped someone in one shot. If they put it in blops 2 id use that all the time, though of course this would never happen.