Here is a question I often receive: “What is or are your top choices for an English dictionary? I’d much appreciate hearing from you!”
That one is easy. My favorite hard-copy dictionary is the American Heritage Dictionary. It is the easiest to read and understand and it has extended etymoloties. None of the online versions of this work carries the etymologies by Calvert Watkins of Harvard. Of course, I use the Oxford English Dictionary but the best version of it is now on line (www.oed.com) and by subscription only.
The most comprehensive dictionary on line is www.thefreedictionary.com. I used it recently in updating our English frequency list and it consistently had words in it that others did not. It also contains Wikipedia articles on some of the more arcane words, but that is OK: it saves a separate search.
Others I use include yourDictionary.com, which I founded. It now apparently has the exclusive rights to Webster’s New World Dictionary by Wiley Publishers. It, too, has the American Heritage Dictionary as a secondary source though, as with the Yahoo version of AHD, it does not carry the excellent etymologies by Calvert Watkins.
For etymologies I rely on Etymonline by Douglas Harper, just up the road from me in Pennsylvania. This etymological dictionary is great for Romance language borrowings. For native English terms the Oxford English Dictionary is still the best online source. I use other hard-copy etymological dictionaries in my library but they are mostly in foreign languages.