Goblet of Fire is my favorite. Favorite movie. Favorite book. I want my wedding to look like The Yule Ball. I love almost everything about it. The twins are at the height of their wit, Hermione actually seems to like Harry and Ron again, the champions are all well cast, and the challenge sequences are fun and exciting.
The plot does have the one tiny hiccup of irrelevance. Barty Crouch Jr. could have just shown up as MadEye, thrown Harry a smelly old shoe portkey, and bob's your uncle. No need for the whole Tri-Wizard Tournament thing. Details!
I also love the Quiddich World Cup, the tiff between Harry and Ron, Moaning Myrtle and Neville, Draco as a ferret, and of course the grave yard scene where we get to see Voldemort - full force - for the first time. Harry's parents' appearance at the end gets me every time. And Cedric asking to have his body brought back - sigh.
I could gush forever about Goblet. I have nothing bad to say about it. It's magical.