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X-Men: First Class (primarily 1962)

X-Men: Days of Future Past (largely 1973)

X-Men Origins: Wolverine (primarily late 1970s)

X-Men: Apocalypse (1983)

X-Men: Dark Phoenix (1992)

X-Men (early noughties)*

X-Men 2 (mid-noughties)

X-Men: The Last Stand (mid-noughties)

The Wolverine (2013)

Deadpool (2016)

Deadpool 2 (2018)

Logan (2029)

You should prefer watching them in the order of release

Or else all the fun would be ruined