Claremont's X-Men run is without a doubt the greatest sustained run on a superhero comic in history. It is so far above everything else that I can't even think of what might be second.
Claremont/Byrne was the highlight of that run and for years they made a lot of really great comics together. And both did very fine stuff after.
Claremont had a decade on X-Men after Byrne left, including as people have pointed out the very fine work with Jim Lee and the classic God Loves, Man Kills. Byrne had FF, Superman, Alpha Flight and more.
Two really great creators.