Grassy_Aggron Classic Sonic has no chance, honestly. But Modern does. Flash has more muscle in the arms, but Sonic has a stubbornness that is unmatched by any but Azelf (the Being of Willpower, ironically XD).
Sadly you're correct. Classic has absolutely no chance in heck. The versions I'm using for this fight are as stated.
The most modern Sonic (which includes any manga or anime perks), which means homing spin dash, punches, kicks, anything from brawl (lol), and the ability to turn into Super Sonic at least once during the fight (for roughly one minute, tops).
For the Flash, I'm using Barry Allen. Only because he seems to be the most "classic," but formidable Flash people are acquainted with. More notably because of the recent cartoons, too.
Both are fast, but what Sonic lacks in speed in comparison to the Flash is, kind of like what you stated, his stubbornness. Sonic has more "natural" abilities, like his spin dash, claws, reflexes; which will probably cause a problem when the Flash is even trying to lay a finger on his quills during a spin.
This was an interesting fight to write and calculate, by far. I think y'all will enjoy it when I'm finally ready to post.