The Age of Apocalypse

This is by far the best story arch that I have ever read. For four months the regular x-series was replaced by their age of apocalypse counterparts. The story itself is a result of Xavier's death in the past at the hands of his son, David a.k.a. Legion. The age of apocalypse is the present that comes into existence after this event.

X-Men Chronicles 1, 2

The Chosen

X-Men: Alpha

The Astonishing X-Men 1-4

Factor-X 1, 2, 4

Generation Next 1-3

Weapon-X 1, 3

The Amazing X-Men 1-4

X-Calibre 2, 3

Gambit and the X-Ternals 1-4

X-Men: Omega