X1, by far. It sets up a nice, consistent tone not just for the action but for the overall feel of the movie. The main theme is sad, but coupled with determination. That they might not win, but they sure as hell are gonna try. And the gentle tones surrounding the vulverable Rogue add a nice touch of beauty and gentleness. It sends shivers through me but for all the right reasons. What's upsetting is that the music didn't stay consistent throughout the 2+1 films.
Other famous series known not only for their story but their music are Star Wars, Indiana Jones, and Superman (ok, so John Williams did the music for all of these) that always retain the basic melodies at least when it involves the main story of the film, or the main chars and with each movie added other themes to specific characters or character pairings but in each case the basic melody was always recognizeable and remembered. Even though a different person scored the music for Superman Returns, he kept William's recognizeable melody and theme.
X3 had a great film score but didn't fall back on the basic "X-men" theme which I thought added a nice touch as the melody doesn't sound happy and exciting but in a way touches at sadness with a bit of determination. The Phoenix-related music had this aura of mystery, wonder and fear but for critical scenes like Alkali Lake, Whirlpool of Love is too loud and rushed. That scene and the music should have been tender, and touching. Gentle. With a hint towards the end that something MAY be wrong. Instead of this loud obnoxious symphonic crescendo and another crescendo at the final end that just blows up in your face and switches to Xavier screaming. And the melody itself doesn't have a consistent tune but instead seems to break up and change suddenly 1/2-way like 2 pages of different songs spliced together.