Hart, now 24, joined wealthy rivals Manchester City in 2006 from League Two Shrewsbury Town after United passed up the opportunity to secure his services.
"I could have bought Joe Hart for 100,000 so we all make mistakes," Ferguson told BBC radio in an interview to be broadcast later on Monday.
"If you look at the England goalkeeper situation for the last 20 years, I would think he's easily the best," added the Scot who signed the now-retired Dutch keeper Edwin van der Sar in 2005.
United have struggled to find a replacement since Van der Sar retired last year while Hart has been a rock in goal for Premier League leaders City.
Ferguson, who turned 70 in December, has been in the hot seat at Old Trafford for 25 years during which he has won 12 top-flight titles, two European Cups and five FA Cups. He has spoken of staying on for another three years.
"I think you always want to go out on a winning note," he told the BBC.
"Hopefully we can do that. I don't know how long I can last now but if my health stays up I don't think another two or three years would harm me."
