The Sinister Six premiered in Amazing Spider-Man Annual #1 (January, 1964) by Stan Lee and Steve Ditko, and included 6 of Spidey's biggest, baddest foes (at the time...all also created by Lee and Ditko!).
This was the original gathering of this bunch, and certainly a tough task for Peter Parker to face...
Let's break the six down, one by one...
One of Six: Electro
Electro quickly branched out as well, fighting Daredevil (even founding his own group of villains, the Emissaries of Evil with Leap-Frog, the Stilt-Man, the Matador and the Gladiator) and he later joined the Frightful Four (a group of villains that gathered together to fight the Fantastic Four including the Wizard and Trapster, as well as some later on this list) as well as battling against Captain America's partner, the high-flying Falcon!
Two of Six: Vulture
The Vulture was a more elderly villain, set in his ways, and stayed in New York to face Spider-Man mostly, though occasionally he fought others like Daredevil and the Punisher, and even facing off against the Beyonder in Secret Wars II and Rusty Collins of the New Mutants!
Three of Six: Mysterio
Mysterio worked as an unnamed assistant to the Tinkerer (a foe Spidey first fought in Amazing Spider-Man #2), but continued to bedevil Spider-Man over the years using tricks like a fog that nullified Spider-Man's Spider-Sense and other tricks to make Peter Parker doubt himself. Like most Spider-Man villains, he also ended up facing Daredevil...and the one and only X-Man as well!
Four of Six: Sandman
Sandman started his career menacing Spider-Man, but soon branched out, facing the Human Torch alone in Strange Tales, joining the Sinister Six, then being a founder of the Frightful Four (along with the Wizard, Trapster and Medusa). Sandman even battled the Hulk, and then became friends with the Thing...trying to turn good for a time (even ending up as a provisional member of the Avengers) before going back to crime.
Five of Six: Kraven the Hunter
Kraven the Hunter fought Spider-Man with mucho gusto, and also battled with Iron Man, jungle lord Ka-Zar, Daredevil and the Black Widow, Spider-Man's foe the Man-Wolf, Tigra the were-woman and more, considering most heroes as big game, and it was later revealed Kraven was a member of a 1950s version of the Avengers! Kraven even took on a nobility in facing Spider-Man in Fearful Symmetry: Kraven's Last Hunt by J. M. DeMatteis and Mike Zeck.
Six of Six: Dr. Octopus
Otto Octavius was a young kid, bullied by those around him, including his father, so turned his attention to science, becoming a respected nuclear physicist, engineer and researcher... ....and while using a creation of his own (mechanical tentacles) to control a reaction, it went wrong in Amazing Spider-Man #3 (July, 1963).
Still, all these villains will come back and face Spidey again, but they have proved their reach expands through the entire Marvel Universe and beyond!