Man this is a tough one. The easy answer is Avengers.
But that opening scene of Nightcrawler in the White House and the raid on the mansion where Wolverine escapes with the kids gave me nerdgasm goosebumps I didn’t think were possible. X2 had a very Empire Strikes Back feel to me. It’s been a while since I’ve watched it but it has remained in my top 5 Superhero movies even after all these years. It’s a shame the series was never as consistent.