Personally, I think that there is no clear winner but some online dictionaries are really worth mentioning: Dictionary.com, Merriam-Webster’s Learner’s Dictionary and Cambridge Dictionaries Online.

• Use Cambridge Dictionaries Online if you need both British and American pronunciation written using IPA symbols. You can also use it as an idiom finder.
• On the other hand Merriam-Webster’s Learner’s Dictionary will provide you with useful pictures (see “car” entry) and allows you to hear american pronunciation.
• Finally, Dictionary.com is handy when you need more information (synonyms, etymology,..) because it will show you entries from different dictionaries like Webster’s Revised Unabridged Dictionary, The American Heritage® Dictionary of the English Language, WordNet and so on.

And what’s your favourite online dictionary?