Byrne's X-Men 141 one of the most famous covers in the history of comic books
Mainstream is the word for John Byrne's work; he has done all the major comic book characters but his style is so easy on the eye that it is forgotten how consistent it is . It flows easily and is comic book enough to work on pretty much any material. His work developed into writing and his run on the flagship comic for Marvel-The Fantastic Four is extraordinary.
His tenure on Superman was also very interesting. Disliked by many I find his artwork easy to scan and its simplicity hides a great compositional sense. This is exemplified in his cover work.