The Green Goblin and his successor were some of the worst enemies of Spider-Man.
Green Goblin (Norman Osborn)
The first Green Goblin.
Through some unknown reason, Norman Osborn became indebted to the Kingpin. In exchange for a loan, Osborn agreed to fund Spencer Smythe's Spider-Slayers program to destroy Spider-Man. If that failed, his company would always be at the Kingpin's disposal. The latter situation was what took place.
To destroy the Kingpin, Osborn hired a petty crook named Jason Philip Macendale, and outfitted him with new weaponry to kill the Kingpin as his new enforcer: the Hobgoblin. Hobgoblin failed and betrayed Osborn, becoming one of his deadliest enemies.
Kingpin later ordered Osborn to create chemical weapons for him, and this was soon leaked to the press and printed in the Daily Bugle by J. Jonah Jameson. The Oscorp Board of Directors was going to replace him, and Kingpin was still ordering the weapons. While testing a new gas, there was a leak and Oscorp exploded. Osborn was missing and presumed dead. However the gas gave him great strength, as well as insanity. A new personality emerged from the old, and equipping himself with gas-altered Hobgoblin gear, came to be known as the Green Goblin. The Goblin kidnapped the Oscorp Board of Directors and placed them on trial for hypocrisy and destroying Norman Osborn. He was defeated by Spider-Man, and the Goblin personality was seemingly destroyed, with Osborn remembering nothing.
Months later, the Goblin re-emerged after the Kingpin threatened the life of Norman's son, Harry, in exchange for knowledge of the Hobgoblin's identity. He attacked the Hobgoblin, and captured him and Felicia Hardy, stealing hisTime Dilation Accelerator. Before he could kill them, he was defeated by Spider-Man, but managed to use the Accelerator to escape into a dimensional portal. He later re-emerged and miniaturized the Accelerator, using it to spy on Spider-Man and finding out that he was Peter Parker.
He attempted to kill Spider-Man at the George Washington Bridge, but failed. He kidnapped Mary Jane Watson, and while battling Spider-Man at the bridge, Mary Jane vanished into a portal. Spider-Man, believing her dead, vowed revenge. The Goblin's Accelerator malfunctioned, and he was rammed by his glider into the portal, with the device destroyed and thus no chance of his return.
Norman later managed to make contact with Harry from Limbo, and helped make him the new Green Goblin, unsuccessfully using him twice to destroy Spider-Man.
An alternate version of the Green Goblin appeared in another universe, working alongside the Hobgoblin as a henchman to the Kingpin and Spider-Carnage.
Green Goblin (Harry Osborn)
Having felt betrayed by his friend Peter Parker (for "stealing" Mary Jane Watson away from him), Harry was contacted by his father, the original Green Goblin, from limbo. Harry became the Green Goblin and mounted a revenge campaign against Peter, before being defeated by Spider-Man and captured by the Punisher- having by then really gone insane.
Harry was taken to Ravencroft, but escaped when he learned that Peter was marrying Mary Jane, and forged an alliance with Silvermane to destroy both Spider-Man and the Kingpin. He attacked the wedding with help from Spider-Slayers and the [[Scorpion (Marvel Animated)|Scorpion], and tried to marry Mary Jane himself before being stopped by Liz Allen, who admitted her love for him. His hatred cooled, he returned to Ravencroft to get more help.