In his column for The Sun, Redknapp wrote: "Van der Vaart has already proved a great buy - but he is not in my top three transfers during my time as a manager.
"At No 1 I'd put a player only Bournemouth and Port Vale can probably remember, John Williams.
"I paid just £20,000 for John, the best money I have ever spent as the big centre-half was truly magnificent for me and the Cherries.
"He stopped goals, scored goals, made goals and I'd say he made my managerial career, too.
"At No 2 I'd put Paul Merson - and he didn't cost Portsmouth a penny in transfer fees.
"Aston Villa even paid a big part of his wages because Pompey couldn't afford them.
"Merse arrived at Fratton when Portsmouth finished fourth the previous season and he gave the entire place a lift with his personality. attitude and ability.
"I'd say Merson was one of the main reasons Ports-mouth gained promotion to the Premier League for the first time, he scored 12 goals and was the catalyst for an era on the South Coast that few could have dreamed of.
"Third, I'd put big John Hartson who joined me at West Ham from Arsenal in a deal worth £3m.
"Many said I'd paid over the top for a player who was already over the hill - but I sold John two years later for £7m.
"John battered defenders, he was virtually unplayable because of his aggression and his heart.
"He kept the Hammers up when relegation was looming.
"I think it's fair to say they are three honest professionals who changed clubs they played for - and the direction of my own career."