Reigns is the best ex Shield by by many miles that's for sure.
I would still say Ambrose. He just isn't protected like the other two.
Ambrose is so patchy though. When he's really trying and being booked as the right type of character he's good and he gets over well but way too often he's in goof mode and works matches like he doesn't give a shit. Some of his performances can be almost embarrassing tbh.
Case in point: last night's IC title match. It wasn't only a bad performance by him, but it seemed like he intended to put on a bad showing. After bitching on SCSA's podcast about pitching ideas to Lesnar and he not giving a fuck, surely he looks even worse by putting on a very lazy performance.
Reigns, even when he is in a dire situation like his match against Taker, or being punished like his losing streak after his suspension, will always put on a very good effort and usually is the better performer in his match.
Could you imagine the state of that match if he was booked with Undertaker?