Yeah, Ol' F3 has appeared occasionally , but so have some of the other Pre-Hero monsters. What really separates this book from the pack is that name. Fin Fang Foom is the ultimate example of the fun Stan had naming the endless parade of wannabe world beaters.
That sense of fun is one of the biggest reasons I'm a pre-hero Marvel collector. Stan really let loose, particularly in the early days, before the monster formula wore thin. Add to that amazing, epic stories from Steve Ditko and especially Jack Kirby, the real Master of the genre.
Strange Tales #89 is arguably the single most popular Pre-Hero Marvel, and Fin Fang Foom is certainly the most popular monster.
Not only that, FFF has always been more rooted in the Marvel universe. Although used briefly, for example is one of the main characters in Steve Gerber’s 1990s series "Legion of Night" (which features non-superpowered characters previously appeared in various Gerber scripted titles – very original).