I voted Zack Ryder, and I'll tell you why he's the most promising one to breakout:
First, as somebody had stated above me, Tyler Rex is a failed experiment. He simply doesn't fit the atmosphere in WWE. Maby if he goes to TNA, he has more success. Who knows.
Chris Masters, his problem is different. If you've watched his recent matches, you could see he look pretty much like a main eventer, since he's huge, has a whole lot of strength, and has a few nice moves like The Dropkick he learned. However, in the first time he signed with the company, he didn't have as much moves, and he was even worse than Mark Henry in the ring. The company seems to remember that time, ignore his improvement, and book him to job on. I don't know what's the reason behind it, but it's no doubt something political, so it's unlikely to change.
As for Zack, he has everything to be an upper-mid carder. He has the potential charisma and the mic skills, which you can easily see if you watch his
relitively new show. He also has some very good wrestling skills. His problem is his gimmick, but this can always be changed in many ways. By the way, his new show seemed to get him much more popular both with the fans and the superstars, which is a good thing for him. There is a high chance of him getting a push very soon, and I really hope to see it. It's way over due.