Among the various dictionaries I have ever used to this day, I strongly recommend the one called "Longman Dictionary of Contemporary English." with its online edition.
The first reason for using it: free of charge. You spend no money but get the most of it. All you have to do is click and link to its Web page -- http://pewebdic2.cw.idm.fr and there you go. You are then free to use it anytime and anywhere.
The second reason for that -- its plain and precise explanation on words and phrases. Some problem occured to English users when it comes to looking up the English-to-English dictionary for the word they don't understand is they often have hard time catching the meaning of the explanation of the word. This has caused frustratoin and disinterest in learning English. But for the user of LDCE, you can easily understand the meaning depicted in the explanation on each word. Take "AIDS" for example, the dictionary gives you a very easy and understandable way to learn the word like "a very serious disease that stops your body from defending itself against infections, and usually causes death." See how simple it is. You only have to have 2000 words in English in your mind to understand the most of the words and their meanings included in this big dictionary. No sweat to learn it.
One day I hope there will be a PDA version of this dictionary, so I can use it wherever I go instead of sitting at my computer for it.