The finest dictionary I have ever used is the American Heritage College Dictionary (Fourth Edition).
Given its size, you will find that this is an absolutely amazing resource. For several years I have kept two other dictionaries on my desk and without exception the American Heritage provides better definitions, more useful information, and just overall more help in understanding words and their proper usage.
If you just want one, this should be it.
American Heritage College Dictionary (4th Edition)
There is an abridged paperback version which I also use and it's also very good (especially for its size).




