The question is unanswerable because you've posted it anonymously. If you live in the UK you'd be best with the Oxford Dictionary. If you live in the US there are any number specialising in American English. If you live in Australia the Macquarie would be your best bet. And if you live in India I recommend एक हिन्दी शब्दकोश
Personally, I like Merriam Webster, but it really depends on the convenience and what you are using it for.