But it wasn't a weak title when it was being published in the Silver Age! It was a very good one. That's why it's held in such reverence today.
Your comparison is off base though. While Fantastic Four may indeed be a second or third tier title these days, this is being determined by the buying preferences of today's thirteen year old customers at the local comic shop. But these are not the collectors bidding thousands$ for high grade early Silver Age comics!
The collectors paying the big money for Fantastic Four 1 are doing so for two reasons. First of all, that was the issue that launched the Marvel Superhero Universe and revolutionized the history of comics. Secondly, the Fantastic Four title was Marvel's flagship through the Silver Age. That was where the canvas for Marvel's Universe was being painted. That's where the majority of Stan Lee's most creative thinking was displayed, and that's where the major character introductions that shaped the Marvel Universe occurred, e.g. Sub-Mariner, Skrulls, Dr. Doom, Inhumans, Galactus, Silver Surfer, etc. While the Amazing Spider-Man title was a good read as well, Spidey's supporting cast of Doc Ock, Vulture, Green Goblin, Kraven the Hunter, J. Jonah Jameson, Mary Jane Parker, Gwen Stacey, etc. were hardly characters playing lead roles in the grand panorama of the Marvel Universe.