Uncanny X-Men #239 (1988) Mister Sinister
This issue features the 2nd full appearance and the 1st cover of Mister Sinister. It is also the 1st cover appearance of Madelyne Pryor as the Goblin Queen. Finally, it is a crossover in the "Inferno" story arc that affects multiple titles. This book is graded 9.2 NM- with white pages by CGC.