1. Monty Python and the Holy Grail (the first time I watched this I laughed so much that it literally hurt my ribs; one of only, I think, two times that happened)
I knew that was #1, but thinking further I honestly couldn't name many outright comedies; I've watched tons of films with a lot of comedice elements (including, for example, most Disney and Pixar animations), but I wouldn't really label those as being pure comedy films in the way that the above is.