I haven't really heard of anyone seeing it, so I would suggest Ben Hur. Only because very few people (if not only my dad when he showed the movie to me) have mentioned watching it. The movie had a MASSIVE production. It was one of those movies from 60 years ago where they wanted something larger in scale than most movies made today, so they had like, at least 500 fully costumed extras on screen at any give time. Although people seem to have forgotten about it.
Big Lebowski was hilarious, I think it is more "cult classic" than obscure though.
Another one I would suggest that very few people have seen would be "Grave of the Fireflies". It is a anime movie about the death of a brother and sister after their village was destroyed in a WWII air raid (the film opens with the death of the brother, so no spoilers are in effect). It is quite depressing, I even cried a bit when I saw it.