De Bruyne, 33, has announced he will leave Manchester City at the end of the season.
Writing for the Daily Mail, Souness said: "In 25 years' time, when the name Kevin De Bruyne comes up in Manchester City conversations, they will be talking about him as one of their greatest ever players and a major part in the success of the glory years.
"I believe he was the best midfield player in the Premier League for a good five years. In time, supporters will reflect on him saying: 'You should have seen him play'.
"He suffered an initial set-back at Chelsea when Jose Mourinho sold him to Wolfsburg. He had the talent but Jose obviously felt he didn't have the mentality but, when he came back to England, Manchester City saw a player who had everything.
"Forgive me, but as a former midfielder, I do believe that unless you've got the correct balance in the middle of your pitch you're not going to be winning anything.
"In recent years, Manchester City have had some truly outstanding, world class midfielders in Fernandinho, David Silva, Yaya Toure, Bernardo Silva and now Rodri but De Bruyne may be the best out of all of them.
"For me he has been world class. He could have played for Real Madrid, Barcelona, Bayern Munich, Paris St Germain or any other team you care to name."