It really does depend on case basis, so this one might need elaboration.
Game Universe: Knuckles
Before he was turned into the dumb-strong archetype, this guy was the GOAT. He has a badass theme song and one of the most conventional ways of dealing with things: fists. Not only that, he has a responsible air about him too, which is one of the traits I tend to gravitate towards heavily. The day he uppercut Sonic so hard he fell out of the super state & then gave that glorious wide grin was a memorable one, while for him, it probably was Tuesday.
Comic Universe: Scourge
I tend to like villains, so how about combining Sonic's wittiness & self-confidence with Hazama's cruelty & sharp tongue, give the guy a cool jacket & then give him the center stage? You get the self-proclaimed king, not to mention a villain way too good to be stuck in a comic. In particular, it was the comic dub I saw on YT that absolutely sold me on this character.
What little I saw was absolutely glorious when it comes to villains not to mention rival characters. While I feel Metal Sonic, Knuckles & Shadow as rivals worked for the stories and the games they appeared in, Scourge himself to me represents the perfect foil to Sonic himself.
Honorable Mention: Shadow
He's the Sonic franchise's representative of the Broken Home, Broken Soul-archetype of characters, and the way it was handled in Shadow's case throughout his initial character arc (SA2, Heroes, '06) was very good for the franchise it was made for. However, what ultimately brings this guy down for me is not only the lack of consistency when it comes to his characterization throughout the franchise, but how as a result of having to tone down the dark themes usually associated with the archetype in order to fit the Sonic franchise, his story does not reach its full potential.