I don't have any of those dictionaries, but I can tell you that when I have worked with Cambridge and Oxford books, usually Cambridge focuses more on the academic English, whereas Oxford goes for a more general use of the language. Again, it might be my perception. Longman is algo a good dictionary, and so is Collins. One tip I can give you, if you want to go beyond word definitions, is to buy Practical English Usage by Michael Swan. It is a guide with a super complete grammar explanation and a complete guide of most common vocabulary problems.