I once would've said unhesitatingly that ROTK was the best film, but now, having watched them a few more times, I'd go with TT. For my scene from the films, I'd probably go with either:
1. Theoden's pump-up speech and the Rohirrim's charge. I have never seen anything in film that reliably gives me chills like that.
2. "You shall not pass." Heck, pretty much the entire Moria sequence was just unbelievable scene to unbelievable scene, and to have this top it off was mind-blowing.
I decline to pick a quote. Too many brilliant ones in the books and too many impeccably delivered ones in the films.