The Sinister Six
The first Sinister Six appeared in Amazing Spider-Man Annual #1. Doctor Octopus united five of Spider-Man's most dangerous enemies together. After some like Doctor Doom and Green Goblin rejected, Doc Ock was left with The Vulture, Mysterio, Electro, Sandman and Kraven the Hunter. Later incarnations replaced some with other villains. Other versions of the group include the Sinister Seven and the Green Goblin's Sinister Twelve, the largest gathering of Spider-Man villains.
In the 1990's Spider-Man series, the Sinister Six was was initially composed of Doctor Octopus, Rhino, Shocker, Scorpion, Chameleon and Mysterio. The latter was eventually replaced by the Vulture later in that series.