This is one of those posts that will forever get updates...
Cover of The Uncanny X-Men 141 (Jan, 1981). Art by John Byrne and Terry Austin.
Many of these have been posted here in the past, but I am going to try to collect them all in a single post! Let me know if you learn of any others in the comments section and I will add 'em!
I think this one counts...
I love this X-Men; Days of Future Past commission by Arthur Adams!
Dork Note: I have always wondered how John Byrne came up with the idea. Now I don't have any insight beyond speculation. However, seeing the splash page from Iron Fist #10 done by John Byrne in 1979 the root of his idea is apparent.
Rob's Room Note: I also found this What If? cover from 1978