I really have to divide mine into bands, rather than ranking one-by-one.
1. X-Men First Class
X2: X-Men United
Deadpool
2. X-Men: Apocalypse
X-Men: Days of Future Past
3. X-Men
X-Men: The Last Stand
4. X-Men origins: Wolverine
The Wolverine
First Band: I loved the newness of first class, as well as the fantastic actors. Also Kevin Bacon's Shaw was a fantastic villain. X2 had some amazing scenes, such as the White house opening and the Woverine/Deathstrike fight, which really drew me into the movie. Finally Deadpool, well what can I say, I've seen the Deadpool/Colossus freeway interaction about 20 times and I still laugh my ass off every time. (C: Do you have off switch? D: Yeah, It's right next to the prostate, or is that the on switch?)
2nd Band: By the slimmest of margins these movies go to band 2. DOFP was fantastic, esp the quicksilver scene. The main thing that bugged me about it was the sheer amount of characters that were killed off from FC, especially Emma Frost, as she is a major player in the X-men universe in general. Apocalypse was great too, and I think the critics have got it way wrong in their scores. For me there were just a few things (Like Havok's death) that felt pointless.
3rd Band: The first and third X-men movies for me were on about the same level. I've never really understood the vitriol aimed at X3. My theory is that people were bummed by the absence of nightcrawler, who was the breakout star and scene stealer in X2. I enjoy these films, but compared to the above they feel just a mark below. Also X3 had a crap version of the Juggernaut, who is my favorite villain/slight antihero.
4th Band: Whilst I enjoyed watching these movies, I didn't really feel they added much to the overall universe, and the first one really sold out deadpool.