Keown wrote in the Daily Mail: "His match-saving performance against Borussia Dortmund proved that once again he is up there with the best in the world.
"From a technical point of view, he has tremendous reach and reflexes and he is fantastic in one-on-one situations, coming out of his goal at a rate of knots. He is so brave too.
"But it is his maturity that sets him above others. After the defeat by Real Madrid last month, where he was culpable for the third goal, Hart was criticised by his manager Roberto Mancini for speaking his mind.
"But he was taking responsibility and Mancini should be proud of that."