Sully is the easy and obvious choice, not only very good but he did it over a long career. I'm struggling to think who else stands out from the rest, plenty who have had good but relatively short careers of the 2 to 5 year length and when you are talking all time you would hope there's more. One real blast from the past who is before just about before everybody's time is Billy Stone, good enough to tour Down Under and score 149 tries for the club in the 1920's - on this information alone he must have been useful. Also someone will have to say it but Tom Briscoe's record stacks up well against at lot of the other names mentioned here.
For personal entertainment value I enjoyed watching Super Taff. However as a cult hero there is nobody to match Super Alf, who looked nothing like a winger is supposed to look but managed to get the job done and at times outdid some illustrious opponents.