Shotguns, especially in games with gore or games with ridiculous physics. I was playing WAR FACE when I decided to go into a game as the Medic (with their default weapon being a shotgun), the first thing that I noticed was that not only are they stupidly powerful, the enemies actually fly off their feet, making pretty much every kill to be satisfying.
Borderlands 2 Corrosive weapons (with PhysX), there is nothing better in that game than shooting someone in the face with a Corrosive weapon and watch them slowly melt into a puddle of green goo on the floor, its probably one of my favourite uses of PhysX so far.
Also, Sniper rifles on Battlefield. With bullet drop like that its just so satisfying to nail someone across the map and watching that Marksman bonus pop up. Points galore.<spoiler=Video><youtube=pZ8ozePe3rE>Please excuse the wacky music