From the editorial file First appearance of the X-Men, September 1963: Cyclops, Marvel Girl, Beast, Angel and Iceman, plus Professor X and, in the same issue, Magneto. Hero team and arch-villain arrive together, which few first issues manage.
First appearance (cover) of Scott Summers - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Cyclops - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Hank McCoy - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Beast - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Jean Grey - the earliest appearance on record is a cover, not an interior story. An independent public reference records the same first appearance.
First appearance (cover) of Bobby Drake - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Iceman - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Warren Worthington III - the earliest appearance on record is a cover, not an interior story. An independent public reference records the same first appearance.
First appearance (cover) of Charles Xavier - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Professor X - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Erik Lehnsherr - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Magneto - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Marvel Girl - the earliest appearance on record is a cover, not an interior story.
First appearance (cover) of Angel - the earliest appearance on record is a cover, not an interior story.
The evidence
Claims here come from the story records for this issue, listed under Sources at the bottom of the file. Character hubs with full evidence tables are coming to this site.