Wordweb is best since:
Along with the dictionary it gives you Synonyms and Anagrams among other things. Also you can listen to the pronunciation (although it uses Windows own speech engine).
And you can bring the wordweb to front from anywhere, by just selecting a word anywhere, and pressing a global shortcut: Ctrl + Shift + W.
another vote for wordweb, lightweight, fast and very simple and straightforward interface, no clutter, didn't found anything better than wordweb in past 7 yrs..
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