Well, we have a MCU and a DCEU thread, might as well start one about the original cinematic team up film franchise.
As far as Marvel comics go, I didn't get into Captain America or Iron Man. It was always X-Men and Spider-Man. So least to say I was more than ecstatic way back when in middle school when the first X-Men movie came out. Seeing Wolverine and Cyclops on the big screen for the first time was just amazing. Then came X2: X-Men United, and which was and still one of the best superhero movies of all time.
This franchise has it's ups and downs, but none more glaring of a flaw than its continuity. I'm pretty sure it was the base for the MCU on what not to do with the continuity of their films. 2014's X-Men: Days of Future Past virtually erased the events of X-Men, United, and Last Stand, and now moving forward with Apocalypse as a sort reboot but not really a reboot. Unfortunately, Days of Future Past is probably the last time we're going see the original cast all together in one film. The third Wolverine spin-off will also be Hugh Jackman's last film we see him as Wolverine. Jackman's Wolverine is like Reeve's Superman. He defined the role, and it's going to be tough to replace him.
So far, I rank all the films as follows
1) X2: X-Men United
2) X-Men: Days of Future Past
3) X-Men: First Class
4) The Wolverine
6) X-Men: The Last Stand
7) X-Men Origins: Wolverine