And for that reason alone it deserves a place on this list.
There is much that could be criticized about this movie. But there is so much excellent work done in it as the same time that it cannot be overlooked.
Like the movie Watchmen, Deadpool is a deconstruction of the whole genre. But unlike Watchmen, Deadpool sees joy in the ridiculousness. There are several snipes at genre failures like Ryan Reynold's own turns as Green Lantern (a movie that I actually enjoy) and his previous outing as Deadpool (a movie I did not).
While watching Deadpool you feel like you are inside and outside the film. Deadpool constantly mocks the tropes of the superhero and yet he dives into these tropes with abandon. So you get the thrill of a traditional superhero flick and the laughs of a screwball comedy without compromising either.
The opening credits alone pack in more humor and visual stimulation that most movies.
One of the reasons I think that this is the most successful of the all the X-Men franchise films is because it is a film with a singular vision and voice and it never waivers from it. That isn't to say that all of the choices are good. But you can feel the difference between a movie like this where those involved are passionate about the material and X-Men Origins: Wolverine, where you get the distinct feeling that the production was run like a committee.
Passion goes a long way here because it creates a bond with the fans. Rather than a simple cynical cash grab, the filmmakers reach out in a personal way and invite you into this weird little world.
Deadpool dares to be funny. It dares to be outrageous. It is almost as if it dares you dismiss it as a piece of juvenile fantasy and overlook all of the skill it took to craft this incredibly well-toned story.
Like the movies John Wick and Mad Max: Fury Road, Deadpool knows what kind of movie it is and it never tries to be something else. And even in this insanity we do get moments of real character development and drama, as with Wade's years of torture.
For a full examination of this, see my review for the movie here.
Deadpool would be much higher on this list if it was not weighed down by its own vulgarity. That is danger of a singular vision: when you are right you do something amazing (e.g. Inception) and when you are wrong you crash and burn (e.g. The Lady in the Water). Deadpool is a mixed bag in that regard.
But for what it does right, it has earned the #24 spot on the Best Superhero Movies of All Time.