Return of the Jedi: I never hated anyone as much at the movies as the Emperor threatening to kill all those characters I loved so much (although this was my first SW film on the big screen, they showed TESB a few weeks later as re-run, oddly enough). The ending was so deeply satisfying, very few things will ever come close.
I am not entirely sure but it also has to be among the first 2 or 3 films I ever saw at a theater.

Indiana Jones and the last crusade: Not even a comedy per se but with some of the biggest laughs of crowds I was part of. Especially the "No ticket" scene and when the crashing plane passes by the Jones' car in the tunnel. You just felt that everyone in the room loved the film.

Avatar. Sure there is the 3D and although we had been going to 3D films for 9 months already or so it took it to a whole new level. But it was the whole world you could dive into, to a degree that had not been seen on the big screen for a while. Leaving the theater we had those glances in our eyes and couldn't stop talking about the film for hours. That says enough, I guess.