Here's #5! Click here for the master list!
Fantastic Four #5
This is the big bad voodoo daddy, the first appearance of Doctor Doom, one of the seventeen best super-villains of all-time!!
I don't think there's really much more to say about this issue, created by Jack Kirby and Stan Lee, except to note that it has one of the silliest examples of the drawbacks of the Marvel Method of writing, as it seems pretty clear at one point that Lee says in the script that the Fantastic Four (who are back in time to steal some pirate treasure) steal some pirate clothes. Now, you'd figure the way you'd draw such a scene is the standard routine where they knock out some pirates and take their outfits. However, Kirby took the instruction literally and had the Fantastic Four steal a big bag of pirate clothes. I think it is the goofiest plot point in the early issues of the Fantastic Four (and there are a lot of them!).
That said, come on, it's Doctor Doom!
'Nuff said, indeed!
However, another notable Marvel character DID first appear in the fifth issue of a series, as Ghost Rider first showed up in Marvel Spotlight #5.
OH! How could I forget Maverick, the Sensational Character Find of 1992! He first showed up in X-Men #5.
And I actually feel quite dumb for not noting that the famous "One punch" issue of Justice League was also #5. Remember when that issue was a real collectible? Late 80s/Early 90s was a weird time, man...
I have to mention how Doom actually made an appearance in the fifth issue of Amazing Spider-Man, too, mostly because that amuses me.
Oh, Asteroid M first showed up in X-Men #5!
There's got to be more notable fifth issues out there, I just can't think of any more offhand! Help me out, folks!