What If Vol. 1 #11
The book opens with The Marvel Bullpin as the Fantastic Four. They are fighting a monster who turns out to be a doctor friend they once knew. They search his lab knowing there is a small box emitting cosmic rays.
This discovery leads them to remember their own Origin. A similar box sent by a group called the S people. Cosmic rays bombard them, Giving them the same powers as the Fantastic Four.
Whenever you hear us old timers talk about the good old days, this is the kind of thing we were talking about. This is pure fun, and I bet the Bullpen had a blast making it. Seriously, it is silly fun. Seeing the King as the Thing and Stan take on the leadership role of Mr. Fantastic, absolutely entertaining. Is it the best of the what if stories? No. It sure is a lot of fun though.