Personally I see Roman being the biggest star, Ambrose being close behind, and Seth being more or less a Chris Jericho kind of guy, like one that goes into the main event when needed, but spends his time with high midcard guys(no not RVD).
Roman has the look, and the impact finisher that gets people off of their feet. His mic skills are not the best, but they can improve. Plus, Goldberg didn't have the best mic skills, but was still over with the crowd because on how he was used. I'm sure Roman will increase on the mic skills, and his in ring will improve too hopefully. He still has the marketable look, and WWE seems to care about that the most, along with promo working at times.
Ambrose has great mic skills, not the best in ring, but he has the mic skills to be the top heel one day. His in ring psychology is great too. The way WWE has been working him as a singles player, by putting him one on one wit da Undataka(holla), and giving him the United States championship, the WWE obviously sees something in Ambrose and will most likely get a world championship reign someday in my opinion.
Seth is the best in ring out of all of them, and his mic skills isn't bad either. He doesn't have the marketable look though to be the top face of the company, nor does he have the mic skills high enough to be the top heel. Still, I can see him putting on great matches, and possibly getting a world title reign in the future, a small reign, but it's still a world title reign.