He'd be up there for sure, mainly because of the handicap of owners. As said, impossible to compare across eras, Hill was backed by a chairman but did change the entire culture, Sillett changed us from permanent relegation battlers to major trophy winners and regular competitors, Harry Storer transformed the club and was just stopped from taking us on by the war.
At the moment it's easy to say Sillett and Hill are out there just because of achievement. Robins going up would indeed be astonishing really - if he won the FA Cup he'd be the greatest by far