American Heritage Dictionary (use for syllabification, definitions, stress)
Although the American Heritage dictionary for syllabification, definitions, and American English stress. They have a leading usage panel under the direction of Steven Pinker. However, this dictionary does not use IPA for pronunciation, instead using another set of symbols and diacritical marks.
Cambridge (IPA pronunciation)
Use the Cambridge online dictionary for IPA based pronunciation and the placement of both primary and secondary stress marks in words. Almost all words have American and English audio files.