Thiem. Yes, he won the USO - with Novak disqualified and Fedal not even playing the event. He also needed some serious choking from Zverev in order to win the title. Other than that, he was supposed to be the second best player on clay - he has zero results to back this up. He's now 27 and given his playing style, I don't think he'll have the playing longevity of the big 3. I wouldn't be surprised if he starts declining soon and I don't see him winning another slam.
Dimitrov, aka Babyfed... His WTF win has an asterisk, even more so than Thiem's USO title. The guy is going for the hotshots and stuff, but that's basically all he can do. He started out as a Federer impersonator, and that's pretty much the story of his career. Never had the game, consistency and mentallity to be a true top player. He was hyped for years and years as the next big thing, even though he always had huge flaws in his game - serve, backhand, not to mention his mentality. I can hardly think of anyone more overrated and overhyped in the past decade.