Which narrows it down to Bellamy, Green, Robinson or Bennett. Thats all.
Using a winning percentage alone is not enough to judge a coach. It's the equivalent of saying Hazem El Mazri was the best player to ever play because he scored the most points. Or that Merritt deserved to play origin because he was the best try scorer for so long. There's a lot more factors involved, with the obvious one being the team itself.
If you look even the tiniest bit deeper at Cleary, his career is very impressive. He took the Warriors to the finals 4/6 years he was there, and would have been 5/6 except they lost points due to being over the cap the previous year (before he started). That record included a grand final. By comparison, the Warriors have made the finals 3/15 years without him. Those stats clearly indicate that theres a far deeper cultural issue at the Warriors, and that Cleary was able to overcome that to build success.
You then look at Penrith. Before him coming along, Panthers had made the finals once in 8 years, narrowly avoided the spoon and had salary cap issues that make our club look like a well-oiled machine. Within 2 years, he re-vamped the roster and built to a Top 4 team and a pre-lim final. Yes, he had a crappy 2015 with an almost reserve grade side for most of the season due to a horrible injury run, but the evidence is there. And that rebuilding is something we will need over the next years too.
He is by far the best coach on the market, and better than half the coaches currently employed. To say he's a level above Mary is the understatement of the century; he's in a whole new league to McGregor. Not signing him would be a monumental f**k up.