I can't decide, so I'm just going to offer my opinions.
Marth - Marth is not Sonic's hardest match up. Period. It's true he's really tough, as many Sonics are used to seeing the one-note style of SH Double Fair, and don't understand the kind of options Marth's FH brings to the table in higher play. That just means most of us don't know the matchup as well as we thought we did. I will say that I believe he is tougher to handle than MK though. Yes, I said it.
Wario - Strong candidate. Bite wrecks SDRs like nothing. That means Sonic has to rely on his natural mobility. Strong mobility in the air > good mobility on the ground. Wario makes it very hard for Sonic to defend, a true bane since Sonic is bent on trying to apply pressure to the opponent, or at least occasionally trying to land camp when he needs to find an opening.
Game & Watch - G&W is such a dumb character. Aside from the fact that he indeed does have a MU advantage over Sonic, being comfortable while fighting him is really more of a matter of how to deal with his moves while still being able to counter him from a defensive position. Chances are you'll just run into his moves if you try to rush him at every opportunity, which is unfortunately what a number of Sonics are probably doing.
Lucario - The only real problem here is killing. Next.
Olimar - I'd be biased and say that Olimar is the most likely to be Sonic's true hardest matchup, but it could just all be a test of patience. The same logic in countering from a defensive position applies here, it's just that Sonic also has to deal with a ******** grab and dumb smash range as well. At the very least, Sonic makes non-Purple Pikmin throw useless.
Peach - Test of patience. It only looks bad when you run into/get shield poked by floating dair consistently. Rough MU, but not the toughest.
Tbh, I don't give a fuck what you all say. I honestly believe Luigi should be in the running. Nair cuts through everything ffs.