The Tigers' return to the top-flight with a second-placed finish in the npower Championship was confirmed on Saturday after they drew 2-2 at the KC Stadium with champions Cardiff and third-placed Watford lost 2-1 at home to Leeds.
When Hull went up five years earlier via the Championship play-offs for what proved to be a two-season stint, it was the first time they had reached the top division in their history.
But Duffen, who was chairman at the time having taken over the club in 2007 before resigning two years later, said: "This was even better than 2008.
"To see Hull clinch promotion automatically, without the need for the nervy, fretful experience of the play-offs, was superb.
"Great credit must go to (club owners) Assem and Ehab Allam and to manager Steve Bruce and his team for toughing it out to the end.
"Mind you, there was plenty of tension in the KC as we waited for the news from Watford. I was anxious, everyone was anxious, but it was worth the wait.
"This was a special moment for Hull to reclaim their place in the Premier League. No-one gave them a chance of winning promotion at the start of the season but they have had such a wonderful campaign."
It was Duffen's first time back at the ground since his acrimonious departure from Hull, having been invited by the club's Egyptian owners, who took over in 2010.
"Being at the Cardiff match and winning promotion at the KC was really poignant for me," Duffen said.
"For me to be part of Hull's big day was fabulous. It was a really nice gesture by the new owners and I appreciate it."