In my 30+ years as a fan it's Schofield by a country mile for me.... As others have said, as a half, he had everything. A real threat ball in hand, vision, creativity, long and short kicking game, pace and strength and alright defensively - unfortunately for the majority of his time with Leeds, he didn't have the supporting acts around him to make us successful.
Unfortunately, the way juniors are coached and the way the game is played, I doubt we'll see a half with such a complete game come through in the British game.
Harris at his peak was exceptional (god knows how many Tissot watches he must have for his MOM awards on sky), but I don't feel he had it all like Schofield did. Harris would have been found out eventually, he was brilliant at what he did, but he wasn't as unpredicatable as Schoey - Schoey would just turn it on regardless of field position or what was happening in the game and make a break, chip over and regather etc.... unplayable.
McGuire has been amazing in the current set up, and has been the perfect half to play alongside Sinfield and Burrow, but he's not got the all round game Schofield had.
If you could drop any player into today's environment, with the right attitude (something Schoey didn't always have) and today's training, sports science etc, Schofield would be the better than Thurston IMO.