1. X2
2. X-Men: First Class
3. X-Men / The Wolverine
4. X-Men: The Last Stand
5. X-Men Origins: Wolverine
X-Men 1 and The Wolverine are equal to me and they both always swap between number 3 and 4 on the list and I can't decide. The Wolverine definitely would have been number 3, but the third act brings it down a little, plus I still really like X-Men 1 but it has dated slightly and feels low budget and very TV at times, so they are both number 3 for me.
X-Men 2 still remains the standard for me regarding the XM films and First Class was almost number 1 on the list, but when the film goes away from the Magneto/Xavier storyline it's SLIGHTLY less interesting because some of the secondary characters are REALLY underdeveloped, Darwin was useful, Angel was useless, Havok and Banshee were ok, January Jones' wooden acting as Emma Frost, I didn't care for Moira MacTaggert, Azazel wasn't in it as much as I wanted and Riptide didn't even speak. The film combined the X-Men Origins: Magneto script with the First Class script and the Magneto script is definitely the stronger part, so that stops it from being number 1 on the list for me.
X-Men: The last Stand to me is definitely not as bad as some people say it is, but it's definitely one of the weaker ones. I LOVED Kelsey Grammar as Beast and the make-up was great, the action scene in Jean's house with the telepathic battle between Jean and Charles was great and the Golden Gate Bridge was a great moment, but it does have major problems that everyone has already pointed out.
As for X-Men Origins: Wolverine, I can't add anything that hasn't already been said: too many characters, just felt like another X-Men film than a Wolverine film, origin story rushed, the adamantium experiment scene was MILES better in X2, love story with Silverfox bad, TERRIBLE CGI, dated action with too much wire work, Deadpool just destroyed, crap script, farm scene is rip-off of Martha and Johnathan Kent, Danny Huston nowhere near as good as Bryan Cox and so on... The only things I liked were Hugh Jackman, Liev Schreiber and the first 10-15 minutes with that war montage title sequence and Ryan Reynolds as Wade on that opening mission and that's it.