Flash wins, also “Sonic positions himself as "speed perfected," emphasizing his constant growth and the mastery of his abilities.” Flash’s capable of literally the same thing. Flash constantly grows more powerful over time and has stated that he has mastered his abilities.

Flash’s faster, he accomplished Inaccessible, Immeasurable and Irrelevant Speed in a Year, his first year as the Flash. While it took Sonic 15 years to travel through time on Little Planet. Both get faster as time goes on, but the Flash has a huge lead when against Sonic. But going by this crossover, it won’t have a winner and be likely a tie. But if there is a winner, I’m going with the Flash.