The railroad gun in Fallout 3 was one of the most satisfying weapons I've found. I loved the effect and the sound of it, and I especially loved how it pinned body parts to the wall. After a particularly successful melee, I would have half a dozen severed heads spiked to the walls and ceiling all around me. How can that not make you smile?
Basically, if it makes a big awesome noise and has some effect other than making blood come out of the person you're pointing it at, I'm probably into it. I'm a great fan of overkill, and if my gun can kill someone, set them on fire, fling their limbs, boil their eyes and serve them to me with some fried onions and spanish rice, I'm going to marry it.