An almost impossible question to answer.
The biggest problem is that certain people of certain ages can only comment on their particular era.
Also, comparing players in different positions is sometimes tough. And finally players who played in particularly successful and dominant sides can perhaps appear better than they really were.
That said, having a recollection of roughly 1990 onwards and having a season ticket at Saints throughout that period, I would say off the top of my head:
1. Darren Lockyer
2. Andrew Johns
3. Paul Sculthorpe
4. Andy Farrell
5. Keiron Cunningham