My go to dictionary; I don't go anywhere without stuffing it into my backpack. This is a must have for any writer, no matter what they write.
I know of no other dictionary that provides what American Heritage provides: expert linguistic advice on usage, an account of the word origin, and thousands of "word histories." To top it all off, the dictionary records each definition in order of usage, rather than the normal "which came first."
If you write, you need a copy.