It depends on several things: what you want to use it for (looking up unfamiliar words, usage guidance, etymology or some other purpose), your level and comfort with English, how you feel about non-standard usages and what dialect you are interested in. The big daddy of them all -- and indeed reliable -- is the Oxford English Dictionary, but that is probably overkill for you. (Most individuals would only access it through a library and not have a copy of their own.) Personally, for a more modest but still reasonably compact reference, I like Merriam-Webster's Collegiate Dictionary for US English (although it usually covers specific BrE usages too), and Chambers 21st Century for UK English. The latter has its idiosyncracies, including a tilt to Scottish usages, but I rather like that.

As a U.S. native speaker, I grew up on "the Unabridged," i.e. the Merriam-Webster New International Dictionary, Unabridged. In fact my parents owned a copy. Nevertheless... and despite acknowledging the Oxford English Dictionary as THE authority... I would like to put in a good word for the American Heritage Dictionary, online at ahdictionary.org The American Heritage dictionary is a medium-sized dictionary. I own a print copy of it. I like two things about it. a) I feel that the definitions are EXCEPTIONALLY clear, insightful, and well-written. b) In some important cases, this dictionary has usage notes, and whenever it does I find the usage notes to be EXCELLENT.

http://www.macmillandictionary.com/us I usually use the macmillan.

The Oxford English Dictionary is good old standard one. The Oxford Dictionary of Current English is good for those learning the language currently. The Merriam-Webster English Dictionary is well known. A well-known, good thesaurus is Roget's Thesaurus and/or Roget's Thesaurus in Dictionary Form. These can be found online and/or purchased for reading devices such as IPad or Kindle.