I personally am very fond of the American Heritage Dictionary. I own a print copy, and the first dictionary I go to online is the no-cost online version, ahdictionary.com. Two reasons I like it are that I find the definitions to be exceptionally clear and well-written, and when there is a usage question, I feel that they have excellent usage notes.
The print edition is larger and heavier than most "desk dictionaries" but manageable, and the price is reasonable.
Like all English dictionaries, it seeks to record actual usage--"descriptive, not prescriptive." However, the "usage notes" then present intelligent opinions, like "The stigmatization of 'ain't' leaves us with no happy alternative for use in first-person questions. The widely used 'aren't I?,' though irregular, was found acceptable for use in speech by a majority of the Usage Panel as long ago as 1964, but in writing there is no acceptable substitute for the stilted 'am I not?'"