hello, even though I rarely try to speedrun levels unless it's for record attack, I usually tend to play Metal Sonic since he technically is the fastest character in the vanilla roster and the fact that he can cheese through entire sections with his hover (that is if he built up enough speed beforehand), I've also tried Sonic once and he was surprisingly faster even tho he doesn't feel like it.
As for tails I've never tried but he could be a good choice as well.
For modded characters, I usually go with X Sonic because that is his strongest suit (when I don't screw up a zip dash haha)