The Dutchman left the Red Devils despite the Wembley triumph, bringing a positive end to two tough years at Old Trafford in which he was heavily criticised because of his side's underwhelming displays.
The 65-year-old was sacked after the final and was replaced by Jose Mourinho, and he stated on Monday that United is likely his last job in coaching following a brilliant career in which he won La Liga with Barcelona, the Eredivisie with AZ, a domestic double at Bayern Munich, and three league titles and the Champions League with Ajax.
However, Van Gaal ranks his solitary trophy at United as his best due to the testing relationship he endured with the media in England.
"Winning the FA Cup with Manchester United was my biggest achievement," Van Gaal told De Telegraaf.
"I spent the last six months of my time there on the scaffold, with my head in a noose. The English media were responsible for that.
"The pressure was inhumane at times and I had to stay true to my philosophy and keep motivating the players. When you still manage to win the FA Cup under such pressure, you are proud of it.
"Also because we lost a player to a red card in the final and were playing with 10 men in extra time. That makes the FA Cup a fantastic trophy for me. We faced so much resistance."
Under Mourinho, United currently sit sixth in the Premier League - 12 points behind leaders Chelsea.