My username would seem to suggest Venom...and while it's true that he's one of the best transitions, the be-all, end-all for me is ULTIMATE PETER BENJAMIN PARKER.
In my opinion, Spider-Man was always at his best in his younger years. He was new at the costumed crimefighting and it was harder to keep that identity secret. Ultimate Spider-Man really personifies these traits in the character. Not to mention that his mouth is bigger and smarter, and he never runs out of things to say. His inner monologues ARE Spider-Man, I don't care what anyone says...
Worst? Is there a worst? Perhaps Demogoblin? Who the hell thought that MJ + Goblin = Good idea!?!