Here's #18! Click here for the master list!
New Mutants #18
This was a difficult choice to make, as the decision came down to this issue and the first appearances of two great, if relatively minor, characters/groups - Mary Marvel in Captain Marvel Adventures #18 and the Guardians of the Galaxy in Marvel Super-Heroes #18.
Both Mary and the Guardians would be quite reasonable choices if there was not a more prominent book at the number, but I think that, ultimately, the first issue by Bill Sienkiewicz stands out more than either of those two issues (particularly Mary's introduction - I mean, come on, people - who among you instantly knew where Mary Marvel first appeared?) - as New Mutants #18 is the type of book that I think this countdown is about - important comic book issues that people remember as the issue number. Offhand, people can tell you when Sienkiewicz started on New Mutants - it was a landmark issue.
On Moon Knight, Sienkiewicz had already gone from a Neal Adams knock-off to an extremely innovative artist, but it was in the pages of New Mutants that Sienkiewicz would take that style and expose it to a much larger audience, and therefore establish himself as a comic artist superstar.
Would he go on to do better work than New Mutants?
But for a time there, Sienkiewicz was one of the most popular artists in comics while also being one of the most critically acclaimed artists in his comics, for his unique and innovative approaches to storytelling and character design.
Years later, his run on New Mutants would be one of the few runs on the Top Runs list that was clearly dominated by the artist, not the writer (although Chris Claremont's scripts during their run were well-regarded, as well).
It certainly is not a no-brainer, but I think New Mutants #18 is the most prominent comic book at the number 18.
Fantastic Four #18 featured the debut of the Super Skrull, so you could mention it as a prominent #18, as well!