Which dictionary should I buy?

I recommend you the Cambridge Dictionary.
They have the:

Phrasal verbs
Idioms
Essential American English learner´s
Essential British English learner´s
Advanced Learner´s

It comes with a CD and it shows the Thesaurus. What is it?
It shows the structure and hierarchy of the word. You know that it is much easier to learn the words in context, right?
Also, it brings visual dictionaries, I mean, some words come with a type of picture dictionary. For example, a part of a plane links you with a picture page where you can see a plane and its parts and so on.

The best part is that they have online free dictionaries and you can search in all of the dictionaries, listen to the british or american accent, check the thesaurus...so, I am a huge fan of them.

Check it for yourself, click here.

By the way, last class with Wil motivated me to post this here. So, Wil, if you are going to buy a dictionary, you should go to advanced, ok? Phrasal verbs and idioms are complementary ones. You don´t need to buy them in just one way, you can program your shopping. It is just a tip and thanks for providing me so great classes! Go ahead man!