"There's always a different threat with different players," said the England international to the Manchester Evening News.
"Raul in the Champions League is one of the hardest I have to play against. He doesn't play against you, he plays away from you. He asks you to come and mark him or let him hurt you with his passing.
"In training, John Hartson was ridiculous," he added. "You'd train with him and get eight studs in your ear. On a Friday, he'd attack you from all angles. Kevin Davies is difficult to play against - very physical. But there's not one player in particular who sticks out."