The 'best' dictionary is always a matter of personal judgement. If possible, go to a bookshop offering all these dictionaries, and look up several words/phrases that have given you problems in the past. See which definitions and example sentences are clearest for you. List the features of each (for example: illustrations, special topic entries, coverage of phrasal verbs, idioms, words easily confused, collocations, usage notes, etc), and think about which are important for you.
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