Based on my review of the features of these three dictionaries, I would recommend the Longman Dictionary. It seems to be the most functional and informative, and it is specifically designed for ESL students (e.g., ALL definitions use a pool of only 2,000 words, which will make sure definitions are clear). There are a lot of notes to help guide study. And it pays special attention to the difference between registers (e.g., how one WRITES something as opposed to how one SAYS it in person, with friends, etc.). All are good dictionaries. I think the Longman is the most "friendly" to ESL learners of English.

The first one is the only one I know don't think that makes it the best though. I agree with the first commit...also If you want a good online dictionary go to dictionary.com...it is really helpful to me even and I speak English. It has audio for every word plus its easy to switch back and forth between thesaurus and dictionary.