For sure the LOTR series. I mean, sure, some things were vastly changed, but the cinematography was gorgeous and for the most part it was true to the books, if not the spirit of them.
To a lesser degree, Harry Potter. Some of the movies were great, some were so-so.
The Last Unicorn. *Loved* that as a kid. Beautiful animation, kept pretty much to the book.
The Hobbit (cartoon), loved it as a kid, and while it was a bit different than the book, it got me interested in reading. :)
Excalibur, loved it as a kid watching it with my Dad, seemed to me that it tried to be true to the myths (even though some things were changed) but was also kind of gritty/real.
Pooh, I just realized that all my favorite books are fantasy and they're either too prohibitive to make a faithful adaptation, or they did and it totally sucked. (LOTR being the exception) :(
Also, my pet peeve has been with the movie adaptations of the Marvel franchise. Iron Man and Spider-Man movies being the exception, I have to say the X-Men movies totally sucked!! They totally ignored or mish-mashed stories and characters, or made them stupid. Gah! So frustrating!
Err.. and I guess for non-fantasy books adapted into movies: Henry V (with Kenneth Brannaugh), Othello (with Lawrence Fishburne), and The DaVinci Code. (can't think of anymore right now)