IMO, choosing a great GK also depends on your playingstyle, because if you play offside trap your keeper will have different needs than one that doesn't receive many one on one situations.
If you play offside trap, your keeper's GKO ability must be higher than his GKS:
Keeper must be specially good in:
* Handling
* Reflexes
* One on one
* Positioning
If you don't play offside trap, you must try to get/train a keeper skilled in shot stopping mostly, skills must not be "that" high, but many skills are involved on it, keeper must be good at:
* Handling
* Reflexes
* Rushing out
* Aerial Ability
* Positioning
* Jumping
* Decisions
Note that, even if handling is used in both situations, i don't consider it that important: this is not real life, and GKO engine has it's weakness, for instance, keepers doesn't need to hold the ball because there's no second chance goals, just tipping the ball away is enough, and Corner Kicks doesn't happen after the keeper tips the ball away!