Oxford University is famous for creating one of the biggest English dictionaries (with 600,000 entries).
More importantly, their experts have also created the Oxford 3000. This is a list of the most useful, and most-frequently used, words in English-speaking countries. According to these experts, once you learn these 3,000 words, you can understand 80-90% of English conversations, websites, newspapers, etc.
These 3,000 words also allow you to understand every word in Oxford Advanced Learner’s Dictionary, a special dictionary for students who wish to attend an English-speaking university. The newest edition includes 60,000 words and 109,000 examples.
The OALD includes a lot of special features that you can’t find in other dictionaries — like synonyms and collocations. You can also download an app version on your smartphone that allows you to speak the word you want to look up (in case you don’t know how to spell it — which is a common problem with English words).