So my initial thought was Quicksilver as well, Speedster vs. Speedster, and the two have some in comic history from DC/Marvel crossovers. The problem is, Flash consistently, and canonically, wins those. So it needs to be someone who isn't going to be instantly destroyed by Flash's insane reaction times and hits. So... Would Silver Surfer be able to handle him? Or is that going too far in the other direction?