The attacking midfielder has hailed the experienced goalkeeper, while also taking the time to praise Diego Lopez
Casillas lost his starting berth to Lopez last season after a hand injury, but is widely expected to return to the starting XI under Carlo Ancelotti.
However, even though Isco has nothing but praise for Casillas, he feels Lopez will prove to be a serious rival for the 32-year-old.
"For me, Casillas is the best in the world, having won everything in recent years," Isco stated at a press conference.
"They are both keepers who have proven themselves to be huge talents over their careers. Diego was fantastic last year.
"I think it's great that there's such healthy competition between the posts."
Madrid open their 2013-14 Liga campaign with a home match against Betis on Sunday.