I know lots of people say it's either gold or Marcellus Wallace's soul -- which they base on the band-aid on the back of his head, a location where some old legends claim one's soul can be removed from the body, but which actually is only there to cover up a scar on the back of Ving Rhames' neck...
...but the truth is, there's nothing specific in the briefcase. Whoever watches the flick can imagine whatever they want to be in there, but it's merely what Alfred Hitchcock referred to as a "McGuffin" -- a non-specific term which denotes a plot element which propels a story forward and/or acts as the goal of the characters. So really a person can say whatever they want is in there and there's no way it would be wrong.
Then again, there's also no way they'd be right, because there's nothing in there. 