Over the years we've always considered the Green Goblin to be Spider-Man's Arch Enemy but ever since Dan Slott took over writing SM, he tried to make Dock Ock his Arch Enemy. In some ways he's the Anti- Spider-Man considering how he has a lot of similarities to each other with only difference that Peter had a loving aunt and uncle while Otto had an abusive father and an overprotective mother.
On Green Goblin's side...
1. He killed Gwen Stacy
2. Knocked up Gwen and Lily
3. Buried Peter's Aunt May and almost recreated Gwen's death except this time with MJ.
4. Took control of SHIELD and recreated into Hammer.
5. Forming his own Goblin cult.
On Dr. Octopus's side...
1. He accidentally killed George Stacy.
2. He formed the Sinister Six
3. Almost destroyed earth's population so he would live on in Infamy.
4. He mind switched Peter's mind and as slow burn, left him to die in his own body.
5. He nearly mind wiped Peter's memories and bastardized his legacy.
Who do you think is his Arch Enemy. Goblin or Dock Ock.