I don't think anyone here thinks that Ramos is the best defender out there. He is clearly a "good" defender, he has good pace, good technical ability, strong in the air (both defensive and offensive set-pieces), and he's good at distributing the ball.
When one looks at a player, you have to take everything he offers into account. And Ramos offers more to his team than 99% of CBs in Europe. He's world class because of it. If he was really some big liability, do you think Real wouldn't spend big to replace him? They clearly would have by now.