Which English dictionary should I buy?

I am studying English as a Second Language at the moment. Recently, my teacher gave me an advise that I should buy a monolingual dictionary to expand my vocabulary, pronunciation,grammar, etc. But I wonder which dictionary that is suitable for me because I find at least six dictionaries for advanced learners on the... show more I am studying English as a Second Language at the moment. Recently, my teacher gave me an advise that I should buy a monolingual dictionary to expand my vocabulary, pronunciation,grammar, etc. But I wonder which dictionary that is suitable for me because I find at least six dictionaries for advanced learners on the internet. They are:

Oxford Advanced Learner's Dictionary, 9th Edition

Longman Dictionary of Contemporary English, 6th Edition

Collins COBUILD Advanced Learner's English Dictionary, 9th Edition

Cambridge Advanced Learner's Dictionary, 4th Edition

Macmillan English Dictionary for Advanced Learners, 2nd Edition

Merriam Webster's Advanced Learner's English Dictionary, 2nd edition

Could anyone tell me the difference between these dictionaries? And which one is more useful than the others in IELTS exam? Thank you in advanced
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