There are many ways to say that something is realy cool in Spanish.
If we want to use slang we should notice that the translation of 'cool' is different in every country.
For example:
Que guay!
Que chido!

Vocabulary:
Spain: 'guay', 'de puta madre', 'cojonudo', 'chulo', 'chulisimo', etc mean cool
Mexico: 'chido'
Venezuela: 'chevere'
Argentina: 'padre'