It really is impossible to pick favorites. I love movies and the moment I try to pick one or a few, I immediately second guess and feel like I'm leaving stuff out, etc...
But I can clearly say that the best experience of seeing a movie in a theatre was Pulp Fiction. I saw it with a good friend of mine when it had just come out. I had seen Reservoir Dogs and knew who Tarantino was, so I was looking forward to it. But seeing Pulp Fiction for that first time just blew me away. The out of sequence storytelling was just awesome and each section had such an effective tension-and-release, I was in awe. I remember being in a great mood after it was over, just thinking, damn that was a great movie.