Fastest I don’t know because he simply plays really fast on a lot of his stuff. Most difficult Syn solo according to the man himself is M.I.A. but maybe for us it would be Exist.
Most.diffocult song ever probably The Oddysey by Symphony for the fact that it’s technically challengin but it’s also a 24 minute song. Pure based on how hard stuff is to play it might as well be a racer X/Paul Gilbert, Jason Becker/Marty Ftiedman/Cacophony, Dream Theater, Guthrie Hogan, Allan Holdsworth song