I really like the American Heritage dictionary, both in print and online. I own a copy of the third edition in print. Online, I use ahdictionary.com. I find its definitions to be very clear, accurate, thoughtful, and well-written. Compared to m-w.com, ahdictionary.com seems less cluttered up with slow-loading advertising, and less infested with cookies. In cases where there is a usage question, I find its usage notes to be excellent. I really prefer it to m-w.com. In principle, I would have to say that the Merriam-Webster Third New International, Unabridged, is the most complete and authoritative dictionary of American English, but I don't think their desk dictionaries are anything special, and while m-w.com is free, the actual online edition of the Unabridged requires an annual subscription fee of $29.95. OxfordDictionaries.com offers a specific "English Dictionary (US)." It seems to be accurate and reliable with regard to US usage.

I'm using two dictionaries but their main dialect is British. (Longman and Cambridge) You can use the Merriam-Webster dictionary for American English.