...the super-strong monster Thing (Benjamin J. Grimm), leader and elastic in mind and body Mr. Fantastic (Reed Richards), the fiery Human Torch (Johnny Storm) and the hard to see Invisible Girl (Sue Storm, later Sue Richards). Four friends who premiered in Fantastic Four #1 (November, 1961 by Stan Lee and Jack Kirby).
....the original X-Men were created "in the sensational Fantastic Four style" and the Avengers formed when Rick Jones' call for help with the Hulk to the FF was redirected to Iron Man, Ant-Man, Wasp and Thor!
The Fantastic Four had met the Hulk and Ant-Man before the Avengers even formed, and when Spider-Man was looking for money, he went to try to join the Fantastic Four!
Friends of the Fantastic Four
Medusa (as a member of the Frightful Four, along with the Wizard, Paste-Pot Pete and the Sandman, in Fantastic Four #36 in March, 1965 by Stan Lee and Jack Kirby), then followed by Gorgon in Fantastic Four #44 (November, 1965, also by Stan and Jack), and the whole royal family of Inhumans (including leader Black Bolt, Karnak, Triton and Crystal) in Fantastic Four #45 (December, 1965 also by Lee and Kirby).
Thor and elsewhere, and graduating to a few series of their own, like a 10 issue run in Amazing Adventures #1 to #10 from August, 1970 to January, 1972, to their first self-titled series with the Inhumans #1 to #12 from October, 1975, to August, 1977 and oh so many more!
Klaw killed T'Challa's father, T'Chaka (Fantastic Four #53, August, 1966).
Black Panther worked with the FF against a few foes quickly, ending with Psycho Man in Fantastic Four Annual #5 (November, 1967)...before moving on, meeting Captain America and soon joining the Avengers!
...oddly, with both those series ending elsewhere...and the Black Panther getting more series as well...
Foes of the Fantastic Four
But, that wasn't the end of Namor's tale...or the trouble he would cause the Fantastic Four, in more ways than one. While battling the FF (and soon after, the Avengers, with the Hulk, no less), the Sub-Mariner also freed Captain America from his prison in ice (allowing Cap to join the Avengers!).
But, Namor wasn't without a title for long, ending up in one with fellow Fantastic Four foe, Dr. Doom (that of Super-Villain Team-Up).
But, Dr. Doom existed before that series started...facing off against the Fantastic Four in Fantastic Four #5 (July, 1962, by Stan Lee and Jack Kirby), and had been a thorn in the lives of the Fantastic Four even before they went on their voyage to space and received their powers.
Victor von Doom was a college classmate of Reed Richards and Ben Grimm, who was annoyed by Reed's being a better scientist than Doom. After an accident in the lab where he was working (which Reed tried to warn him about), Doom left the university, to head to Tibet, making his armor and then going to home to Latveria to take that country over (all from Fantastic Four Annual #2, September, 1964).
Dr. Doom also made the rounds, working with other villains like the Sub-Mariner (in Fantastic Four #6 in September, 1962), and even facing other members of the Marvel Universe, like Spider-Man (in Amazing Spider-Man #5 in October, 1963) and the Avengers (starting with Avengers #24-25, January-February, 1966 or earlier, as later reports came in)!
...but you can read more on this here, with the title becoming mostly reprints after his appearance, Dr. Doom DID defeat the Marvel super-heroes!
Magneto and odd super-group the Champions were the guest of Super-Villain Team-Up #14 in October, 1977, and a reprint of the two Astonishing Tales battles with the Red Skull filled Super-Villain Team-Up #15 in November, 1978).
All this villainy, where can heroes turn?
Everyone is a part of the Fantastic Four family, from Inhumans like Crystal and Medusa, to Heroes for Hire like Luke Cage, She-Hulk and Ant-Man...that was the charm of the Fantastic Four....it was a party, and everyone was invited!
The family was the core of what made the Fantastic Four such an important part of the readers lives...and, by extension, made the whole of the Marvel Universe part of our family as well.