who would be faster? I'm place my bets on Sonic, because he actually moves at the speed of sound, and while the others are pretty fast, too, I'm not sure if its been implied in any other cartoons that Speedy Gonzales or the Roadrunner can run that fast
It depends on who is writing the story, which version of the characters we're talking about, and what powers they have at that point in time. I don't know any of Sonic's comic history so I couldn't give you much information on that, but so help me God if someone brings up Super Sonic, or even worse, Hyper Sonic well... that'd just be an unfair match.
Obviously Sonic isn't going to stand around smiling like an idiot while Gonzalez runs 10,000 circles around him, and Road Runner is going to give it his best.