As you may know, there are three popular E-E dictionaries available easily: (1) Webster, (2) Longman, and (3) Collins-cobuild. Firstly, the webster has the most vocaburaries among three's. But, it lacks the length of explanation and example expression. If you need the brief explanation of the word you find, and if you prefer the rich database of words, i would like to recommend the webster. Secondly, the longman's merit is.....portable. That is, you can bring it with ease because it is not big and generally it has hard covers. The last is the newest of three's. To my knowledge, it is produced in England so it accepts the British English. Its database is not large, but the length of explanation and example expression is very good and easy to understand. If you need the example expression rather than the richness of words, you had better get this dictionary.