Intense struggles
Characters that would push Sonic to his limits and make him go all-out in order to win, potentially even needing him to go Super in order to win, although the fight would be easy from there.
This is characters with power to destroy large nations and small planets. They can move consistently quick, lagging behind Sonic by a bit.
This includes characters like pre-nerf Collector (base), the Limitless Devourer (not Unlimited Devourer), Inkmatas (full power), Eggyhead (full power, not prime), Thrull, etc.