What version of Sonic are we using? Game Sonic is actually much more powerful than most people would give him credit for, and within his Super Form he'd likely be much faster than Reimu via Solaris bullshit, and he has a lot of pretty good abilities and haxx.

Though frankly, this fight is decided by one thing, Game Sonic has no way around unrestricted Fantasy Nature. Reimu uses it, she floats out of reality itself and becomes untouchable to him. None of his haxx, despite how potent it is, can affect outside of reality to my knowledge. So at the very absolute best, he could maybe just stall the fight for as long as needed, but realistically, that's not going to work.

Reimu's abilities directly target the soul and gradually, overtime, he'll get chipped down until his soul is destroyed or he gets sealed or he just runs out of juice. It would be a very, very long and very much so a fight of attrition, but I'd be on Reimu. Add onto the fact she's likely stronger via Dream World scaling stuff and Sonic's in a bad spot.

ARCHIE Sonic vs Reimu is a very different and much more complicated story though.