X-Men #221 CGC 9.4
1st appearance of Mister Sinister! X-Men and Wolverine cover and story, Chris Claremont and Marc Silvestri
X-Men #221 CGC 9.8
1st appearance of Mr. Sinister, Havok vs. Wolverine cover, X-Men vs Marauders story
X-Men #244 CGC 9.4
1st appearance of Jubilee! Chris Claremont and Marc Silvestri
X-Men #244 CGC 9.6
1st appearance of Jubilee! Chris Claremont and Marc Silvestri
X-Men #244 CGC 9.8
1st appearance of Jubilee! X-Men cover and story by Chris Claremont and Marc Silvestri
X-Men #256 VF (8.0)
1st appearance of the New Ninja Psylocke! X-Men vs the Mandarin cover and story, Jim Lee art
X-Men #266 NM (9.4)
1st full appearance of Gambit (See X-Men annual #14 for first cameo appearance).
X-Men #266 CGC 9.8
1st full appearance of Gambit! X-Men & Gambit story!
Uncanny X-Men #317 NM (9.4)
1st appearance of Blink, art by Joe Madueira, X-Men cover and story, might make 9.6
X-Men Annual #14 VF/NM (9.0)
1st appearance of Gambit in a cameo appearance. 1 week before X-Men #266.