...............BESIDES JOKER.
I realize that Joker will be the primary reply to this question. However, I'm looking for a villain who is not only insane, but who is in the same category from a power perspective... someone who could also go toe-to-toe with GG, or at least hang with him longer than 2 mins.
For anyone not all that familiar with Green Goblin, here are his general powers/abilities: "superhuman strength (lifting 9 tons under optimal conditions), increased speed, reflexes, endurance, and healing rate. Though much slower than the likes of Wolverine, he can regenerate damaged tissue and organs. His intelligence has been enhanced to gifted levels, though at the price of his sanity. ... he is able to maintain his sanity via chemically treated dermal patches. When not impaired by mental illness, Osborn is a cunning businessman, masterful strategist, and highly skilled in electronics, mechanics, engineering and chemistry. The Green Goblin is armed with a variety of bizarre devices." (source: Marvel wiki)