I use the Oxford Learner's Dictionaries: www.oxfordlearnersdictionaries.com
The reason I prefer this for my online dictionary is that they use IPA symbols to show the pronunciation. This means that each sound in the word is represented by a symbol. This is especially useful when working on vowels.
Learn more about vowel sounds vs vowel letters here:
You can also learn the correct syllable stress for words using the online dictionary, both by listening to the recording and repeating, and by looking at the pronunciation guide for the marked syllable stress. You can read more about that here: