I was 9 years old when my mom took me to see this movie in its opening weekend. I just knew that the commercials looked good, but after seeing it, the movie instantly became my favorite of all time, and the series is still my favorite bunch of movies ever (I think I'd put RotK slightly above Fellowship). I read The Hobbit sometime after Two Towers released, I think, and read the Lord of the Rings trilogy the same year Return of the King came out.
I saw it two or three more times in theaters, and The Two Towers was my first ever midnight showing. I've since seen each of the 3 films well over a dozen times each (probably closer to 20), more than any other movie for sure. I still rewatch all of them a couple times every year.
One thing I remember when watching Return of the King in theaters was that I had to take a piss about an hour and a half into the movie, but it was fucking Lord of the Rings. I couldn't just get up and go to the bathroom. So I held it in. By the time the movie got to like the 4th "ending," but then it turned out we still had to see Frodo go on the ship and then Sam go see his kids, I was sure it was mocking me, lol. As soon as the credits started rolling and I was sure it was actually over I darted out of the theater and into the bathroom.