Cambridge online dictionary is a highly recommeded dictionary for high level students. It is divided into various sections. There is an idioms dictionary, phrasal verbs dictionary and more.
For those of you looking for an online English dictionary with audio recording of words, Merriam's Websters dictionary is just the best on the web. Look up words and choose to hear them pronounced in mostly North American accents.
If you don't find a word here, it probably does not exist in the English language. One of the oldest refences on the web. This will be an ideal dictionary for advanced English Learners. It has a thesaurus dictionary for you to match your search with synonymous words.