There are a gazillion dictionaries out there, and some are much more respected than others. Many people think “Webster’s” is the key word, but it’s actually meaningless; what matters is the publisher and its reputation for lexicography. Here’s a roundup on the dictionaries editors tend to favor.
Merriam-Webster’s Collegiate Dictionary. This is the dictionary most often used by copy editors at magazines and books, for its careful lexicography and usage notes. If you are publishing professionally, it’s good to be in sync with your editors, so ask if this is their dictionary of first reference. (When I worked in newspapers, Webster’s New World Dictionary was the dictionary of choice because it tended to add terms more rapidly. But most literary types prefer the other dictionaries listed here.)
American Heritage Dictionary. This is a favorite of many wordsmiths, including me, for its extensive usage notes. In the Fifth Edition, word meanings use quotations from classic and contemporary writers, and etymologies trace some words all the way back to their roots in ancient Indo-European and Semitic.
Random House Unabridged Dictionary. If you’ve got a huge budget and a big bookshelf, this is a wonderful dictionary to have in addition to MW 11.
Oxford English Dictionary. If you’ve got an even huger budget and a bigger bookshelf, go for this 20+ volume dictionary, which includes citations all the way back to Beowulf. This is one for real dictionary snobs: The citations go back as far as the dictionary researchers can trace a word, so you can see how Chaucer, Shakespeare, and Mark Twain used it. It has even been the subject of a bestseller, The Madman and the Dictionary, by Simon Winchester. Many libraries make their print and digital copies of the OED available to cardholders, so you may want to check with one in your area. (Oxford also publishes smaller dictionaries, which are reputable, but not usually preferred by American publishers.)