Hart has grown in stature since replacing experienced Republic of Ireland international Shay Given as number one at the start of the season.
Since then the 23-year-old has not only become established in the team but also become England manager Fabio Capello's first choice.
"It was very difficult for me at the start of the season when I had to decide who to pick because we had two fantastic goalkeepers," said Mancini.
"The reason I picked Joe was I felt he could become the best goalkeeper in England - and he has.
"But he is young. It is only his second season in Barclays Premier League. He can improve a lot. He can be the best goalkeeper here for a long time."
Source: PA
Source: PA