Viduka has faced some incredible players in his time, whether it be at the 2006 FIFA World Cup or during his time in the Premier League and in the Champions League.
But the former Socceroos striker singled out three names when asked by Zelic.
"Physically? I use to have a lot of battles with (Arsenal's) Martin Keown all the time. We were like cat and mouse," Viduka said.
"He'd like to throw in a little sucker punch behind play and annoy you as much as possible so you can get sent off.
"Rio Ferdinand was a very good defender. I use to train against him at Leeds. You'd get past him and suddenly he would pull out this bloody huge foot and somehow get it back of you.
"One of the toughest was a guy called (Roberto) Ayala. He was a little guy (177cm).
"He played for Argentina and Valencia and I played against him in the semi-finals of the (UEFA) Champions League."
