Good reflexes will be my pick. Nothing beats good goalkeeping than having good reflexes.

I admire De Gea a lot as he is able to react fast to situations and make a good save.

Aside from textbook saves, a goalkeeper's ability to react to situations is paramount to a clean sheet

The other options I will look for are good ball control and passing. We have witnessed awfully lots of instances where goalkeepers give the ball to the opponent to score.