Discussion Forum—A Way with Words, a fun radio show and podcast about language

Great question! Just to name a few basic metrics to judge by, the best online dictionary would be free, it would have clear definitions from multiple sources, it would have synonyms and antonyms, it would have illustrations and pictures, it would have an audible pronunciation guide that is accurate and sounds like a normal voice, including alternate pronunciations, it would have etymological references, it would include an extensive list of idiomatic usage, it wouldn't shy away from slang references, and it would provide explanatory sentences. I use many online dictionaries, including the one you mention, but none of them meet the aforementioned standards.

The online dictionary I use most often is dictionary.com. It also has a handy foreign language translator. Otherwise it is pretty much the same as thefreedictionary.com.