Since I am often on line, I usually go to dictionary.com, and use their definitions. The dictionary that they often rely on is American Heritage Dictionary
I use the Oxford English Dictionary since it has so many more entries than any other dictionary. What is more, it covers the vocabulary of Shakespeare and Chaucer, so I find it quite useful. The only real drawback with the OED is that it's so expensive when you buy it. But it will pay its price back in gold.