This is one of those things that I've always wondered about. In most debates, some seem to believe Green Goblin isn't that strong at all because there doesn't seem to be many feats of his strength. Green Goblin and Hobogoblin are generally presented as Spidey's equal after taking the formula, but I never got that same impression with Kraven.
In "Beware the rage of a desperate man", Macendale was given an elixir that bestowed him powers that Kraven once possessed:
Spidey also says Macendale was equal to Green Goblin, if not a bit stronger:
I guess one could say Kraven appeared to be comparable to Spidey in "Kraven's Last Hunt" with the way he easily defeated Vermin, but I always thought that more to do with Kraven not holding-back, if anything. Either way, this formula basically puts Kraven on par with Spidey and the Goblins based on the above.
Lefty Hobgoblin seemed to have the same exact power as Green Goblin:
http://i.imgur.com/3dO53SF.jpg
http://i.imgur.com/6sxPbAf.jpg
The only difference was it still contained the side-effects from the original formula.
Kingsley's version seemed to eliminate those issues and gave him enough power to equal the Green Goblin:
http://i.imgur.com/XCcy3ae.jpg
http://i.imgur.com/QqNUWmb.jpg
Demogoblin Macendale's another that appeared to be in Spidey's league:
http://i.imgur.com/utuxJ4M.jpg
http://i.imgur.com/7W4RKAi.jpg
http://i.imgur.com/J4Rfcab.jpg
So for the most part, there doesn't seem to be much of a difference between the Goblin's in strength. The only real discrepancy here is Demogoblin. He's completely outmatched by the newly enhanced Hobgoblin, which seems odd if his demonic essence alone was what allowed Macendale to fight evenly with Spidey for as long as he existed. He was even a Ghost Rider enemy for a bit.
Harry seems to be the only one utilizing a different formula:
http://i.imgur.com/P5bOsHD.jpg
http://i.imgur.com/nEIbGvt.jpg
In Harry's mind, this made him more powerful than Spidey, and it would've made Norman more powerful had he finished it. Spidey only says they're "evenly matched", though, and neither gained an upper-hand over the other.
Finally, there's this:
http://i.imgur.com/sqfoBom.jpg
This takes us back to all of the Goblin's appearing to possess similar abilities regardless of the version of the formula they took. I always assumed one Goblin was clearly superior to the others, but I can't seem to find anything establishing this in any of the stories. I guess the real advantage comes down to what type of equipment they're carrying. In an all-out brawl, they're all pretty much equal.
Kraven seems to be right in that same class based on Macendale's display after taking the elixir. He's the one that seems strange in that list, but I like the idea of Kraven occasionally being a match for Spidey if he's enhanced. Hope you guys enjoyed my little analysis on the matter.