Although Ashton admits that both were formidable to play against, Ferdinand had the edge because of his intelligence.
"If I had to play one v one against any defender, I would probably prefer to play against John Terry, because he likes the battle, and so did I, and I felt like I was probably stronger than him," Ashton told talkSPORT.
"Whereas, Rio Ferdinand is the best defender I have ever played against.
"[He was] so clever, so intelligent. Every time I wanted to back into him he wasn't there, he'd just step in front of me and steal the ball like a Rolls Royce then fizz it out to the full-back.
"He didn't get too involved in aerial challenges when he didn't need to. [He was] just a very, very clever defender."