All bow down before the KING! The answer to me is The Return of the King hands down, no contest. But... this answer is strongly biased, because The Lord of the Rings has a capital importance in my life: I read the trilogy in my early 10s and the movies were released shortly after, so I was exposed to the Tolkien universe in a big, big way and it had an equally big influence on me.
Besides, at that time I was a kid/teen living in a lower to middle class family with almost no access to technology, apart from one console with two games and a VHS player with four or five movies, so reading the events of the books and then watching them captured in a such a magnificent way was a magical experience for me back then. When I first got a VHS of The Fellowship of the Ring, I watched that movie literally once a day (for a total of tens of times, more than 50 for sure). Also, I used to go walking around my house every afternoon, while creating stories in my mind with the Middle Earth as the setting and my own character in the center of all the adventures. And as time went by, I started to add more original characters and creatures, and then I left the Middle Earth behind to create my own world, and I even invented my own runic language (a very basic one, but still). At some point, I had so much original content in my mind that I needed to express it in some way, and that's how I started the activity that has conditioned every single one of all the important decisions that I've made throughout my life: writing. And all of that started with The Lord of the Rings, so if there's One Bias in me to rule them all, this is it.
On the other hand, if I use the Force to try to hold that bias back, the result is still the same, but much less one-sided, because Star Wars has an excelent lore, fantastic ambientation and very good characterization with great special effects, and of the eight movies of the main saga that I've watched (I haven't had the chance to watch the last one yet, unfortunately), I've liked them all, including The Return of the Jedi. However, Star Wars in general is not as good in storytelling and dialogs as The Lord of the Rings is. And while a lot of people complain about the slowness of the latter (which is fine, because it's a matter of taste), that's not a problem at all for me, because I can enjoy both fast and slow movies as long as the pace is good, and The Lord of the Rings is very well paced (although The Two Towers is slightly worse than the other two in that regard).
I'm mostly a lurker now.
