I'm a big fan of the "evil opposite number" school of super-villainy, thus my fondness for The Frightful Four. The evil FF has been a thorn in the Fantastic Four's side since FF #36, when the wingless Wizard organized the Sandman and Paste-Pot Pete, all of whom had fought the Human Torch separately, together with the mysterious Madame Medusa to form a new, sinister rival to Reed Richards' famous team:
The Frightful Four posed an ongoing threat for much of 1965, appearing in issues 36,38, and 41-43 causing the FF to temporarily lose their powers, then kidnapping and brainwashing the Thing to fight and attack his own teammates. At the end of the two teams' third encounter, Madame Medusa betrayed the others, and returned to her people, the Inhumans, after escaping an infatuated Johnny Storm.
As the Fantastic four went on to encounter the Inhumans, the Silver Surfer, Galactus and more cosmic scale threats, the Frightful Four seemed less of a menace. While Lee and Kirby would bring back members of the Frightful Four, the Sandman and the Wizard separately, the assembled team would not return until Fantastic Four # 148 with new member Thundra:
My own first impression of the Evil FF is still fixed in my mind as a summertime read of most of the issues that comprised the wonderful Frightful Four/Brute saga of 1976-1977. Fantastic Four # 178, 179, and 182 introduced The Brute, the newest member of the Frightful Four (and evil alternate-universe Reed Richards), in a multi-issue saga by Roy Thomas, George Perez, and Joe Sinnott. The story found a returning-from-a-long-adventure Fantastic Four ambushed by the Wizard, the Sandman, and the Trapster. The villainous trio then hold auditions for a fourth member in the FF's own hijacked Baxter Building as the captive FF watch helplessly.
Eventually, the Fantastic Four "won", but the Brute was able to impersonate Mister Fantastic for a few issues while the real Reed fought his way out of the Negative Zone. Thundra, Tigra, and the Impossible Man were also hanging around the Baxter Building at the time, so it made for quite the epic.
Again, for several years, the Frightful Four kept a low profile, the standard trio teaming with Llyra and Electro on different occasions. John Byrne never used them in his historic run on the book, though the Trapster had the honor of being beaten by the Baxter Building in one memorable short story. The Frightful Four have popped up again and again over the years. After the Sandman reformed, the Wizard enlisted Morrie Bench, the Hydro-Man to take his place, and other members over the years have included Klaw, Titania, Dragon Man, and the She-Thing.
Mark Waid brought back the team in Fantastic Four #513-515, introducing Wizard's estranged wife, Salamandra. The Wizard and the so-called dragon's daughter had a child together years before, the now adult Cole, whom Wizard had experimented on. as a child. He disowned when she turned out normal, but now, Salamandra reveals that Cole has been manifesting powers over gravity and mass.
The story took on a Romeo and Juliet twist when Johnny became romantically involved with Cole, trying to persuade her that Reed can cure her condition. Despite aggressively disregarding Cole's safety throughout, Wizard does come through as a father at the end, coaching a mass-bloated Cole to control her new powers. The Frightful Four have been basically a gang of hired goons since their formation, so this was a welcome and intriguing twist; recasting them as a dysfunctional family mirrors the Fantastic Four's healthy family all the more accurately, and adds some much needed pathos to the rivalry. The evil FF most recently resurfaced in Dwayne McDuffie's run of the FF. Salamandra and Cole were unmentioned, with Titania and Klaw rejoining the team to fight a Black-Panther led Fantastic Four.
Regardless of team composition, we can probably bet that some iteration of the Frightful Four will once again rear their heads in the Marvel Universe.