I have to say The Lord of the Rings trilogy simply because, having adored the books I was petrified at how they might turn out when made into a movie. The Ralph Bakshi movie from 1978 was interesting and back in the day, when I saw it, I was quite impressed, but when Peter Jackson's epic was announced, I was concerned. Could he do it justice, the justice my favourite deserved? Well, turns out that he most definitely could. Jackson's trilogy is as much a treat for my eyes as the books were (and remain) a treat for my brain.
So, what could have been a HUGE case of Lunchbag Letdown, ended up being something wonderful and much loved. :)