Here's #24! Click here for the master list!
This was a tough one. My original pick was going to be Young Magazine #24 (1982), the Japanese magazine where Akira first appeared. Then I thought that was more than a little bit too clever, ya know? I mean, come on, who knows what issue Akira debuted in? It's a magazine, it doesn't even have standard numbering! So that seemed a bit too much.
So that left two choices (in my opinion, of course). Avengers #24, which was the notable end to the Avengers battle alongside Kang in the future. Remember how cool that battle was? And how the princess that Kang could not intimidate into marrying him suddenly falls for him when he does the right thing and then she ends up DYING for Kang? That was so bad-ass, and ended up being the basis of at least two notable Avengers story arcs (one by Roger Stern and one by Gruenwald - okay, notable is a bit of a stretch for Gruenwald's story - but it did give us one of the cheesiest mini-series EVER!!).
However, when it comes to notable issues in comic book history, I think Miracleman #24 stands out. On the surface, it is just a standard good comic book by Neil Gaiman and Mark Buckingham.
However, what it really is is the very last issue of Miracleman EVER!
Eclipse went under around this point, and the rights to Miracleman have been a bit of a (rhymes with blustermuck) ever since, so this issue has become famous due to the inability of people to ever, you know, READ IT!
So yeah, while I have gone from one cutesy answer (Young Magazine #24 1982) to another one (the last issue of Miracleman), I think it works, consarnit!
Other notable #24s include those of Lee/Ditko's Amazing Spider-Man, Kirby/Lee's Fantastic Four, Gaiman's Sandman, Morrison's Doom Patrol and Moore's Swamp Thing!!