A year ago I would have said The Flash. But the recent comic crossover shows Sonic taking Reverse-Flash by surprise by suddenly appearing in front of him. Thawne speculates whether or not he used the Chaos Emerald they were fighting over to teleport, to which Sonic doesn't confirm or deny. If he wasn't using the emerald, that means he speed blitzes Reverse-Flash who is the embodiment of Negative Apeed Force, and by extension Sonic is also potentially faster than the Speed Force itself.