The next X-Men movie after First Class, 2014’s Days of Future Past, is a team up between the young and old casts, and that really works better after you’ve met both groups.
Granted, X-Men: The Last Stand isn’t fantastic, but it’s better than its reputation.
Then bypass the rest of the prequel series and go straight into the original X-Men trilogy back to back to back.
So watching them in chronological order dooms you to sit through a movie you can’t fully appreciate, then endure a movie no one could appreciate.