In chronological order.
Days of Future Past (1981): Classic X-Men story with Kitty Pryde as a major character. Most mutants have been killed off. Sentinels have gone bad and the future doesn't look bright for mutants and non-mutants alike! Takes place in Earth-811.
Kitty Pryde and Wolverine (1984): Great mini-series of a team-up between Wolverine and Kitty Pryde. Pryde gets taken over by Ogun the devil in this journey to Japan.
Joss Whedon's Astonishing X-Men run (issues #1-24, giant sizes 1-2): Amazing X-Men run by Whedon, taking Kitty Pryde and making her an important superheroine. This is a must read!
All-New X-Men: Marvel NOW's All-New X-Men series where Kitty Pryde has become the headmistress of Jean Grey's School for Higher Learning. The original five X-Men from the past have arrived to present day!
Battle of the Atom: Related to All-New X-Men, the original five X-Men from the past have caused trouble, with the future X-Men arriving to the present as well. Major event which will definitely have an impact on the future and past alike.