* Which: This is the most common translation if "cual" is asking a question. For example, "Cual es tu color favorito?" translates to "Which is your favorite color?"
* What: This can be used if "cual" is asking for a description or characteristic. For example, "Cual es el problema?" translates to "What is the problem?"
* Which one: This is used when "cual" is referring to a specific item from a group. For example, "Cual de estos dos te gusta mas?" translates to "Which one of these two do you like better?"
The best way to determine the correct translation is to look at the context of the sentence.