I'm not sure how we're defining "surprisingly amazing" but my vote goes for Robert Horry.
He's the winningest player in the modern era. He was valuable on every team he was a part of, and you can make the case that all of his teams needed him to win.
He's been the league's biggest swing factor in deciding championships throughout his career. He was never the skilled, the most talented, or the most interesting, but he was the most surprising. Whenever the time came he would pull a big shot or big game out of his magician's bag, win a championship, and then fade back into the picture until it came time to win another ring.