The X-Men are legends, but it didn't start out that way. The X-Men debuted in 1963, making a minor splash at a time when Marvel was at the height of its powers. However, X-Men was almost certainly the least popular Marvel book of the Silver Age and eventually was relegated to a reprint book. However, all of that would change with Giant-Size X-Men #1, by Len Wein, David Cockrum, Peter Iro, Glynis Wein, and John Constanza.
Source：https://www.cbr.com/xmen-claremont-era-best-things-marvel-comics/
