It is all relative. Considering I've watched a lot of football since the seventies I have seen some great centres. Comparing them between the five metre rule and the ten metre rule is a bit hard. Of who I have seen under the ten metre rule, outside of Meninga who was able to play under both rules, Fittler and Daley would still be the two best centres I have seen, under that format. The other bloke that is often forgotten amongst centres is Steve Renouf who did some amazingly freakish things on a football field.
Meninga, Miles, Cronin, Rogers, Kenny..... The list under the five metre rule is endless. It is a different style of football and difficult to compare eras. I never saw blokes like Fulton at the peak of his playing career but the bloke is an immortal.
Hodges and Lyon would be two that could sit alongside a lot of others I have seen. Gasnier however IMO was a decent player but more was made of his famous surname than his overall playing ability.