The Return of the King (1980): That's right, folks: 1980. My boss, a huge Tolkien fan himself, loaned me his copy. And I gotta say, I really enjoyed it. I know it probably has a lot to do with timing (i.e., I've seen Peter Jackson's version a couple of times now [1, 2]), but still, this animated ditty has something to offer all Tolkien fans.
Ditty's appropriate, actually, because the original music in this movie is one of the best - if not the best - part. I'll agree with my boss here, Where There's a Whip, There's a Way is one kick-ass orc song! :-) But there are many good ones. The lyrics are excellent, and they create the perfect mood. Something not to be forgotten in all this Jackson fanfare is the huge role that songs play in Tolkien's works. I understand why Jackson didn't go that route, but it makes this movie their perfect complement.
Similarly, since its scope is limited, it can really tackle the ring material in the book. I loved the way they put us right in Frodo's nightmares when he and Sam are resting on Gorgoroth. The whole bloody journey just dragged until you couldn't help but feel for the poor guy.
I found the death of the Lord of the Nazgûl a bit disappointing (Éowyn was pure Japanimation, eh?), but, overall, I really liked this movie. Oh, and who can forget Casey Kasem as Merry! I laughed out loud the first time he spoke. :-)