Fellowship of the Ring was by far my best theatre experience ever. Not necessarily my favorite movie ever... but best theatre experience ever, no question. Partly, I think, because I had no real expectations for the first one, so I was all the more unprepared for how good it actually was. And that Balrog fight... I still think that Balrog is the coolest monster in the history of movies, period. I'm not sure it will ever be topped.
BTW, have you guys seen the production diary videos? They're really cool, on a par with the great behind-the-scenes features on the extended edition LotR DVDs. I'll embed them all here for people who haven't seen them yet.
https://www.youtube.com/watch?v=jqaUk-r8DBk