Ok give me some time since these references were all from years back.
Hadlee by the way was quite clear that Imran was the best of them:
The 1980s saw the emergence of 4 great allrounders in world cricket Kapil Dev Ian Botham Imran Khan and Richard Hadlee. All vying for the title of the best ever allrounder. Now one of the quartet is picked as the best. Its none other than Imran picked by Richard Hadlee himself in this interview...
cricketcrowd.com
Lillee said it in his book where he calls Imran the best in the world around the early 80s.when it was released.
Holding was a Cricinfo interview if I recall.
Ian Chappell said it in a Aussie TV show segment where he was asked to name his top 5 all-rounders, and named Imran ahead of the others in his list (forgot the order but Sobers was no.1)
Border and Greg Chappell said it in TV interviews. I need to find those references again.
Dickie Bird put Imran as the only one of the four in his ATG XI:
Dickie Bird names his greatest all-time Test XI: in pictures
www.telegraph.co.uk
So did Rodney Marsh: