Depends on what you use for ranking criteria, of the ones I've seen it would be;
1- Peter Schmeichel -Advantages ; played behind great teams (treble winners), height and bulk; strengths; shot stopping, marshalling defenders, throwing the ball out to start attacks; weaknesses; getting caught straying off his line, tendency to get chipped
2- Edwin Van der Sar - Advantages played behind good teams, Height, anticipation, calm under pressure; strengths; overall positioning, commits attackers in one-to-one's, encourages defenders; weaknesses, reactions slowed in his later years.
3-David De Gea: Advantages; great with his feet, tremendous stretch/ leap/reach; strengths, great shot-stopper; weaknesses, stays on his line too much, surrenders his area too often relying on defenders, distribution only average
4-Harry Gregg: (different era) Advantages, physical presence was greater than his size would suggest, strengths; absolutely fearless in going for the ball anywhere in his goal area and occasionally the whole penalty box, took no prisoners in one-on-one situations (got away with murder at times) great reach and acrobatic performance, reached shots you wouldn't expect him to get anywhere near; weaknesses; frightened his own defenders with his kamikaze like dashes across the box/going for highballs, average shot stopping, occasionally 'got done by the eyes'
5- Alex Stepney; Advantages; excellent defenders in front of him, strengths; shot stopping, covering his angles, famous one-on-one stop against Eusebio in 1968 European Final; weaknesses some times delayed too long and surrender advantages on positioning.