Johnny Novgorod said:
Shotguns in general. There's something so satisfying about short-range, wide-spread, pump-action blasts.
I can second this, but only in single player. In most multiplayer games, they're about as effective as sneezing if your enemy is more than arms reach away.
Getting more specific, the shotgun in Black. As a matter of fact, all the weapons in Black were enormously satisfying to use. It was a military shooter on the surface, but that took a back seat to awesome explosions and ludicrous amounts of lead flying down range on top of some truly great sound assets. It was like conducting a symphony with an AK-47.
Funnily enough, I also enjoy sniper rifles in games with ballistic drop and shot travel time. Lining up that perfect shot from half the map away and just watching it arc perfectly into your target is a feeling I never get tired of. Granted, I had to fire 4 shots to make the necessary adjustments, but no one is perfect.