Hmmmmmmm toughy.
X2 has so many great moments and really set things rolling for comic book movies, that Nighcrawler opening, the school night attack, Logans weapon X flashbacks, Magneto's escape, Mystique the entire film and Jeans sacrifice.
DoFP had all the things I wanted to see that they couldnt do back then in an X-Men film, combing powers, Sentinels flying in formation with Magneto, bigger power displays, Fassbender and MaCavoy on top form, dat Quicksilver making dose haterz look like nubs and somehow managing to make a complex time jumping plot not unravel into a giant mess.
Still a tough choice but I think DoFP just about wins it just for sheer ambition. (plus an extended cut to come)