As one who has been to Selhurst as a boy in the 40's
starting from the days of Vic Rouse I can definitely say that John Jackson was the best of them all - remember he played in the days where a 'Nat Lofthouse body charge' was legal and keepers were fair game and therefore brave - he was, and in spades.
Of the modern era then Nigel Martyn was consistently good. Julian I am afraid has some way to go to get in the top 4 - his one weakness is making the Command Decision of when to come and when to stay - the most difficult of the keeping arts. So, my top 4:
Jackson
Martyn
Bill Glazier
John Burridge

Palace have been generally blessed in this position (and used to have reliable goal scorers, too: Ronnie Allen, Cliff Holton, Johnny Byrne and then Wright & Bright - when will the next one arrive ? - hope it's soon.)

Bill Glazier was excellent and might have lasted a lot longer at the Palace,rather than make the move to Coventry when he did.
He didn't want to leave but a certain senior player at the club during that time thought that Jackson was better.Bill thought that this guy had a strong bond with the management and rightly or wrongly his spell came to an abrupt end.
Having said that Jacko was brilliant!!