Since "The Mother of Tears" was utter crap, I had to look out for another prime trilogy. My favourite of those listed is probably Leone's Dollar Trilogy, although I'd have chosen Antonioni's "Vitti-Trilogy" if it would be listed. Don't worry, A, there are just too many to get them all together, not even the most "important" ones.
I'm a bit confused about your comments - "Back to the Future" is too mainstream but "Lord of the Rings" not?!