Sonic is fast, but Saitama has shown comparable speed feats and his punch ends any encounter.
Sonic's speed is his defining trait, but Saitama has demonstrated speed feats in the same tier โ he traveled from the moon to Earth in seconds and kept pace with beings who moved at lightspeed.
If Saitama can tag Sonic, one punch ends it. And Saitama's speed feats suggest he can. Super Sonic adds invulnerability, which is the main counter โ but Saitama's serious punch has overcome invulnerability before (his power is designed to overcome any defense).
The meta-question is whether Sonic's invulnerability or Saitama's one-punch principle takes precedence. Given that Saitama's entire character is built around 'always wins in one punch,' the narrative weight favors him.
Saitama's speed feats match Sonic's. His serious punch overcomes any defense by narrative design. His durability has never been tested because nothing has ever hurt him.
Sonic's speed might let him avoid the punch. Super Sonic's invulnerability could theoretically tank it. Chaos Control provides time manipulation as an escape mechanism.
Saitama wins โ his meta-character status as 'always wins in one punch' overrides Sonic's speed and Super Sonic invulnerability in a neutral crossover context.
Saitama also fights
Sonic the Hedgehog also fights