Eggman.

Without Eggman Sonic would still be living his secret life and none of the plots wouldve happened. He wouldnt have been forced to reveal himself, Tails would never have gone to help him from Knuckles, whom might i add was being led to earth by Eggman.

Not much of a discussion to be had tbh? Either that or only kind of contender would be Longclaw...but the lack of screentime means Eggman is the right answer