Roberto Mancini's side have conceded just 24 times in their 27 top-flight fixtures to date in 2012-13, six goals fewer than the next-best total.
"We are so blessed with quality defenders," England international Hart said of his club-mates.
"But Matija Nastasic has been a sensation. For his age (19), he is like a rock.
"Kolo (Toure) is Kolo - you know what you are going to get with him because he gives everything he's got.
"There's also Joleon Lescott waiting to come in and that's not even mentioning Zaba (Pablo Zabaleta), (Gael) Clichy or (Aleksandar) Kolarov, so I think we are really blessed when it comes to defenders."