I go with Ambrose...
Out of the three; Ambrose needs the least amount of polish. His strikes are perhaps where he needs to work the most, but everything else appears to be right in place.
Rollins has been great in the ring for 3 matches thus far and a bunch of brawls, but he hasn't had his time to shine and show his depth just yet. He will continue to impress everytime he steps in the ring and I am sure they are working with him on his presentation on the mic.
Reigns has the ability. He has that athleticism that will carry him through matches, but he has some things to work on in the ring. He sells well for a man his size, but he comes off as timid in some things that he does. Once he has a finalized set list that he is confident in, he will make this awkwardness dissipate and we'll see him start to rise, so long as his mic skills improve. Personally, I am not impressed with him on the stick as his mouth does this Khali-like movement that distracts me from what he is saying. Plus, I am not impressed by someone screaming out the same statement over and over and qualify that as having mic skills; which is why I can't say that Ryback has them either at this point. The best thing going for Reigns is that he has that look that makes you want him to succeed at this. He is someone that the main event could be built around based on off of his looks, similar to how John Cena might not be the most rounded in the ring, but he has that thing about him that makes him marketable.
I believe all three have their own positives and negatives, but they all will see success in the WWE. If I was betting on the now (which is how I voted), I go with Ambrose because he seems to have the least amount that he NEEDS to improve on in my opinion.