It's a hard one for me, really. I like all three quite a bit. Forrest Gump seems to be kind of the cool thing to hate one for a lot of people nowadays, but I still find it to be quite a good film, and something I don't mind watching every once and awhile. The Shawshank Redemption is quite fantastic. Probably one of the most uplifting films of all time. Of course, Pulp Fiction is great, as well, though I hear Mystery Train steals some of it's 'revolutionary' factor in the non-linearity storytelling department (don't know, though, haven't seen it as of yet).
Hard choice really. That was a pretty good year for films. If I had to choose one, I would probably go for Pulp Fiction, but they all were totally appropriate winners for Best Picture.