IMO the best of the X-Men movies. You have a good level of the racial tension between mutants and humans that the first two movies were built around, but you also had the X-Men actually doing stuff.
The movie sports the largest roster of X-Men, with the addition of Beast, Kitty, and Iceman. In fact this is the only movie where all of the original X-men finally make an appearance. (Cyclops, Jean, Beast, Angel, Iceman)
Not to mention it had some of the best promotional art ever.