Here's #9! Click here for the master list!
This was a really tough one. The first issue that popped into my head when thinking of #9 was Avengers #9, and ultimately, that is what I am going with, but boy, did I try wracking my brain to see if I could find a better choice than the first appearance of Wonder Man. However, in the end, I think it just stood out well enough that it edged out a handful of quality alternatives.
As far as comic book history goes, Strange Adventures #9 would most likely take the cake, with the first appearance of Captain Comet. The 1951 issue is a strange aberration - a superhero comic during a time when superhero comics just weren't being launched, and a good four years before the Silver Age began. Yet there he was - a pretty standard superhero. Except, of course, that the good Captain was also the very first MUTANT superhero!
Heady stuff, right?
However, no one really remembers where he first debuted - people barely remember Captain Comet as it is.
Another memorable first appearance is Electro in Amazing Spider-Man #9. This one is a bit stronger of a case, as Electro is probably more famous than Wonder Man, but I dunno - I think the story of the fake hero sacrificing himself in the end, I think Wonder Man's debut is a more famous issue, overall.
Lois Lane made her first appearance as a featured character in Showcase #9, which IS pretty darn notable, except no one, well, noted it.
Fantastic Four #9 was a great issue involving Namor kicking the FF out of their home.
In Quasar #9, Mark Gruenwald spent 8 pages explaining away a continuity mistake in Millie the Model #57.
Any other great #9s?
I don't think so, so in the end, Stan Lee, Don Heck and Dick Ayers' impressive work in Avengers #9 is the top dog!