Prior is the one for me who I look back on and appreciate how difficult it has been to find a replacement. Stewart averaged 34 with the bat as a keeper which is good, but Prior along with Les Ames are the only two who managed returns of over 40 whilst keeping wicket.
Perhaps relevant to the current debate is that both Stewart and Prior were both functional keepers not outstanding ones.
They were def functional rather than natural. You can see how far back we have to go for England to play the best keeper if you exclude Foakes in SL or WI.
Prior was an excellent lower order bat, I don't think myself having seen both he was close to Stewart keeping wise, but better than Bairstow or Butler.
It's subjective I suppose but both made themselves into 'good' keepers. Prior has hampered for his last year with a nasty injury he never recovered from.