Interesting responses. But if Flash is SOOO fast and has such great reaction time, why did he get tagged by Deathstroke? And guys throwing boomerangs and with freeze guns?
I don't believe that Flash has lightspeed reaction times. He gets ambushed and hit too often. I believe that once he gets going everything speeds up too, but he gets hit too often by non-speedsters for me to buy that he'd react to the green light and be atop Xavier before Xavier has a chance to register that the light is green.
I say Xavier wins. For Flash, thought and action may be closer together than the average person, but for Xavier, thought and action are one.