Personally, The Rocky Horror Picture Show.
I knew it was going to be quite the experience before I went for the first time‡ but it was so much fun I had to keep going back bringing friends each time.
It’s still a fun tradition to do though we haven’t done it since last year, we’re probably going to try and go again in a few weeks.
‡ I had seen it many times before going to see it in theaters for the first time.
The Fellowship of the Ring. Probably the other two movies as well but this one stands out in my mind.
The first Pirates of the Caribbean as well. Back before they turned it into a franchise. Such a fun adventure movie.