This is a great question, and probably harder to answer even, than the best player.
I think there are very legitimate reasons to include Bennett in an all time list, but probably not in a current form list.
I think Siebold has worked miracles this year, and should be the coach of the year. But I'd like to give him a few more years before saying he is the current best, which usually means to me "best over a sustained period".
Ditto Cleary, who has a lot of potential and seems good at rebuilds, but let's see if he can find long term success.
So that really leaves 3 intriguing candidates, who all have some question marks.
With Bellamy, his results over a decade are easily the best. The players that go to his teams almost always play at their peak with him. And the list of spine players coming from his systems, Slater, Smith, Cronk, Maloney, Widdop, Munster, Inglis, is remarkable.
If you judge it on results or cranking out champion players, he is the best.
His question marks are obviously his methods. He relied on a blatantly cheating team for much of his success, and horrible abuses of the game rules in on field tactics. You could argue the "best" is also best for the game, and Bellamy may not be.
The real interesting test for Bellamy will be without Slater or Smith, but it is true he passed similar tests in the past, such as losing Slater in 2016 and unearthing Munster, with flying colours.
Flanagan hasn't had the same dizzying results, but has taken a team from a disaster to a consistent performer. Those asking for a coach to prove themselves at a club like Parramatta could just as easily asked 5 years ago, for a coach to prove themselves at the Sharks. And he did.
Players also seem to play at their peak under Flannagan, and the success is coming through in all grades. Looking at players through his junior systems this last few years, like Scott, Ado-Carr, Isaako, Ramien, Bird, Holmes and Brailey, and it seems the Sharks should be able to find consistent future success if they can continue to produce that sort of talent.
Like Bellamy, Flanagan has some question marks over his methods. The ASADA scandal will forever tarnish his record, even though it was very early in his career and clearly didn't result in wins.
The test for Flanagan is to keep up the results of the last few years, without raising suspicions about his methods. If he can do that, he may reach or surpass Bellamy as the best producer of league talent.
Robinson has had remarkable results in first grade as well. It looks like he has a team that anyone could win with, and easy access to TPAs to assemble such teams.
But people forget that the Roosters have not been a super club throughout the whole Politis/NRL era.
They have basically been at the top in just two periods, that might reasonably be called the Freddy era and the Robinson era. That the Roosters were so good with Fittler on the field says much about him as a player, and IMHO he is criminally under rated, despite being highly rated by most pundits.
But after Fittler, the Roosters had all the same "advantages" Robinson has now, but none of the success. Robinson has created a team that is extremely good, several times. He has shown he can pick some elite players and build an elite team around them, and can do that again and again, even when those elites change.
He is the ultimate "fantasy" coach. He can find the two or three players he thinks will best work, recruit them, and get everyone else working with them. If only my "Super Coach" efforts were half as good.
The test for Robinson would be to see if he could do that with less advantages, but it is possibly a test he'll never be asked to do.
Tough question to really answer. For what it's worth, I voted for Robinson. His methods have no question marks, and building up a club is an important secondary skill, but succeeding in the top grade is the main goal, and he has done that.
But I think all three mentioned have a good claim, and Bennett has to be there for all time best, and Siebold for this year's best.
