Number one reason would be: the wholeness. While watching the movies, you don't get drawn away by overly-scripted lines and mistakes and plot holes. It's almost seamless, and for all 12-16 hours of it, you're in a world of your own.
As well as the breathtaking beauty of Lothlorien, the intense Helms Deep scenes, the music, the costumes, scenery, true-to-the-books-ness, good acting, humour, and of course...Legolas
