If by the best you mean who could perform at the highest level then neither.
Sculthorpe was the only one who could cut it against the Aussies and earn their respect.
Before their respective supporters get their knickers in a twist though they both made enormous contributions to their club at Super League level and as a club fan that's what you really need and want, someone who is consistently good and makes your team better. They both did that, not only good but durable - nobody is any good sitting in the stands.