It's also worth noting that "most players" of any character don't have much idea what they're doing and therefore don't matter much in terms of balance discussion. Having a greater understanding of options and spacing tools is a prerequisite for being able to see what's good and bad from a competitive standpoint.
As far as I can tell, a lot of the Melee high tiers stand the greatest chance against Sonic. I can see Peach and Falco doing well, as I imagine they have the tools to negate some of Sonic's most toxic properties. Peach's rate of hitboxes do a crazy-good job of keeping people out of her personal space, and Falco has a gun to threaten Down-B and general stage control everywhere. Sheik and Fox could be reasonable, perhaps, with needles and lasers (especially needles) being able to threaten a camping Sonic, and generally great hitbox placement/duration/speed to be able to swing and miss more safely than most characters.
Toon Link and Diddy may do well, as AGT and general movement speed are both nice for keeping up in the neutral position. Other than that, I'm not quite sure.
Regardless, it's not matchups that people are really concerned with in terms of Sonic--It's the fact that he never actually has to play Smash Brothers. There is nothing okay with Down-B camping being superior to virtually every other spacing tool. In a game where movement is key, the fastest character in the game also has a cancelable (via wavedash) combo tool that clanks with most ground moves, goes into multiple KO options, is impossible to react to when used intelligently, and requires zero commitment or anything beyond an "if x, y" level of thinking in terms of efficient use makes it so that no significant thought is required when playing him.
Note that all the characters I mentioned have rather extreme, rather unique traits that aren't common in the game. All the characters I've mentioned have relatively versatile/ranged hitboxes, and/or great mobility, and/or can threaten an opponent from a full screen away. People are forced to play the game this way because Sonic is able to effortlessly dictate the way matches must be played (or, depending on how you see it, how they must NOT be played). Virtually every other character is left in the dust, and the ones that are not are playing a very specific game that almost no one is interested in playing, either from a casual or a competitive standpoint. It's overcentralizing in a very bad way and does not promote enjoyable gameplay or thoughtful strategy.