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.