Short and simple. The long, drawn out ones usually come off as more goofy than anything else.

My thoughts on this are a bit divided, I enjoy a good bit of carnage in my fatalities so if it can be pulled off right a long fatality can satisfy any need, although the roots of the game(MK1) were all but simple but graphic fatalities that caught the needed attention and publicity that built this game up to what it is today.

Generally, the shorter and simpler they are, the better. There are some exceptions, but not many.