http://www.macmillandictionary.com/ I like this one because of the phonetic transcriptions, word forms and its organization — the different definitions of a word are links;
http://www.ldoceonline.com/ frequency of the words, examples and (sometimes) images, opposites, syllabification, both American and British pronunciation;
http://dictionary.cambridge.org/ level of the word, synonyms and related words and areas of usage;
https://en.oxforddictionaries.com/ phrases and idioms;
https://www.merriam-webster.com/ examples, etymology and first known use of the word, thesaurus;
http://www.thefreedictionary.com/ variety of definitions, syllabification, idioms and phrasal verbs;
http://www.thesaurus.com/ synonym and antonym organized in alphabetical order or by relevance, complexity, register, frequency or length.
http://sentence.yourdictionary.com/ see examples of words in sentences.