The Oxford English Dictionary (OED) is the most extensive English dictionary. The complete dictionary is in 20 volumes.
In the UK, probably the full Oxford English Dictionary.
Merriam-Webster is one of the most popular English dictionaries in the United States, known for its comprehensive definitions and easy accessibility online. Oxford English Dictionary is also widely recognized and used internationally for its extensive coverage of English vocabulary.
The Century Dictionary was created in 1889. It was published as an extensive and comprehensive dictionary of the English language.
The Oxford English Dictionary contains extensive etymological information on most words. However, no dictionary is ever an "ultimate authority". Words of more recent origin or slang words may be treated more authoritatively in other dictionaries.
Oxford English Dictionary
The Oxford English Dictionary is widely considered to be the most accurate and comprehensive dictionary available.
If you mean in the English Language, the Oxford English Dictionary is by far the most comprehensive.
One of the features of a dictionary is to include the pronunciation. The Oxford English Dictionary is one of these, and is generally regarded as the most exhaustively comprehensive.
The & And
I think you could make the argument that the word "pointless", by definition, is the most pointless word in the English Dictionary.
Webster's
Yes, But most likely you will get The Dictionary At A Local Library or Order One online!