I would say Flash by half a hair. Just looking at their builds, Flash can cover more ground in a stride, and he's less likely to get tripped up than Sonic is. Now when the Chaos Emeralds come into the equation, hoo boy... Still, Barry outrun Black Flash for years, with nothing but Speed Force to sustain him... hmm. What do you think gives more of a metabolic boost, a Big Belly Burger or a chili dog?