Be warned, I am in the minority of opinion on some of these.
7. X-Men Origins: Wolverine
There is a good deal of fun to be had in this movie. Some of the action set pieces are great and I could watch Hugh Jackman doing his Wolverine thing all day. But it probably has the worst special effects of any of the movies. It also completely ruins the character of Deadpool not to mention making Wolverine's past needlessly convoluted. The story is also a bit of a drag.
6. X-Men: First Class
Many think that this is one of the best in the franchise. I beg to differ. It is fun to get a very fresh take on the younger version of these characters. But it comes down to my complete and utter annoyance with Charles Xavier in this movie. He is an idealist who never had to enter the real world. Magneto is a holocaust survivor who constantly gets lectured by a rich, ivory tower egghead. This is also the movie that has the least amount of Wolverine in it because of that, a lot of the charm and energy are missing as well.
The first of the series got a lot right. It took the characters seriously and it hired a fantastic cast. Can you picture anyone else playing Wolverine but Jackman? But if you look at it in the context of the rest of the series, it is a bit dull. There are very few really good action set pieces to carry it through. It also has the worst line of the entire series.
4. X2: X-Men United
What was great about this movie is that it built off of the first and added more characters and more layers. Instead of it being simply good vs. evil mutants, it added the human element into it. It did a great job of showing Wolverine as the reluctant guardian and Magneto as a sympathetic monster, but a monster nonetheless. Again, the place where this has its deficits was in ramping up the excitement to epic levels.
3. X-Men: The Last Stand
I am one of the few defenders of this movie. I know that it has its flaws, but there are some great attributes to this movie that place it above the first two:
-the Joss Whedon inspired cure storyline adds fantastic levels of complexity to all sides of the X-universe.
-the killing off of X-Men, while sometimes handled poorly, raises the stakes in this movie in ways that were''t present in the others.
-Ellen Page as Kitty Pryde was perfect. Her scenes, especially against the Juggernaut were some of the best in the entire series.
-Some people couldn't buy into the emotional side, but I was completely in tune with what was happening. I thought it was such a touching and dramatic moment when Jean asked if Logan would die for all of the people there and he replies defiantly and helplessly, "No. For you!"
-I thought the action set pieces were much better than in the the first 2 Bryan Singer films.
2. X-Men: Days of Future Past
I will give my full review later. But this movie was well written, acted, paced, and themed. It did an excellent job of bridging the original X-movies with the First Class cast. Its major deficit is, again, Singer's inability to infuse a constant sense of action throughout. But that is more than compensated for by a very entertaining spectacle.
1. The Wolverine
This, of any of the X-movies, captures the real heart and soul of Wolverine. He is a beast who wants to be a hero. It has some of the best action sequences of the entire series, not because of the special effects, but because they finally let Wolverine be Wolverine. This movie embraces both the killer and the stalwart found in him. The movie doesn't try to be anything other than a straightforward action adventure and it delivers that better than all the rest of the X-Men movies.