Casillas is indeed an excellent goalkeeper, but I think Petr Cech, who currently plays for Chelsea Football Club in England, is the best keeper today. Casillas suffers from instability from time to time, and Cech is not only more talented, but also much more consistent and reliable.
I'm too young to remember Yashin. I grew up knowing that Yashin is a myth, but I never had a real chance to form an opinion of my own. The greatest keeper I had the pleasure to see for myself is Peter Schmeichel.