It's a tough call but I've narrowed it down to a short list of:
Dino Zoff
Iker Casillas
Gianluigi Buffon
Gordon Banks
Ray Clemence
Sepp Maier
I reckon it's between Casillas and Maier. Maier was just so damn reliable with his massive gloves.
Modern keepers such as Ali, Van der Sar, Cech, are good but not in the same league as the above.
What do you think?