Casillas nominated Atletico Madrid's No1 in a Twitter Q&A.
He stated: "It's the million dollar question. If I had to stay with one right now I would say it's Oblak".
Asked about Joao Felix, Casillas also said of Atleti's starlet: "He has a great future. He has talent. Of course, they will demand the maximum. The responsibility for the demand goes with the price paid for him. He has to know that he has reached a club that wants titles.
"People can see him as the replacement for (Antoine) Griezmann."
And on his former Real coach Jose Mourinho, he added: "I have no problem with him. The situations we experienced were ours. We had good and less good moments."