Reigns is potentially the best prospect right now.
Wyatt has impressed me so far, as I didn't believe the gimmick would work at first. I still think they need something adding to really make them click, but they're on the right road. The problem is that it's completely a one-man show, the other two 'brothers' have nothing about them other than size.
Cesaro is talented, but I agree with the others that he's lacking in charisma.
The one I can't agree with is Ambrose. He just seems to be trying too hard with his character, so he comes across as cartoonish and exaggerated. If he could tone it down a little, I think it would be better. Rollins is OK, and they'll flip him face when the Shield breaks up, but he's going to be another Kofi Kingston type guy, good athletic ability, but he's got nothing other than that as far as I've seen so far. Reigns is the one to watch, big guy, strong, aggressive with a good look. He's got a decent bloodline behind him as well, which can't hurt!