When adding sources what is the best dictionary to use for American English.
There seems to be some confusion that the Oxford English Dictionary is somehow for the English of England only.
It certainly is not! The OED covers English not just in England but all over the world. It notably gives pronunciations for both RP and in North American English, and has many, many, many terms that are marked as Australian or South African or this or that.
The OED is an historical dictionary, documenting words and their predominant senses over time. There is no other dictionary like it in the English everywhere.
That said, there are certainly specialist dictionaries that cover particular topic areas more fully than the OED does. For example, the Dictionary of American Regional English, normally called DARE, covers that topic much more thoroughly than the OED does. There are also medical dictionaries with rare terms that have not yet appeared in a sufficiently widespread and frequent distribution as to have yet made the OED.
For example, check out the September 2013 quarterly updates to the OED to see how much they are tracking English in all its guises.
For scholarly or encyclopedic needs, there's probably nothing that can touch the Oxford English Dictionary. That said, I don't have access to it, so all I know about it is what I've gathered from people who cite it; and from what I've seen, I get a similar impression that RyeBread does, namely that it has obscure stuff you can't get anywhere else, as well as a few head-scratchers for common words. My guess is that the vast, vast, vast majority of the material in it is first-rate, but for typical needs, I get the impression it's no better (and possibly a little worse) than any of the major "collegiate" dictionaries: Merriam-Webster, American Heritage, and Webster's New World being the three I'm most familiar with in paper form, and I like them about equally.
I don't mean to rag on the OED, or people who cite the OED, but I also want to say that sometimes I get the impression (and I must stress that this is both subtle and purely subjective) that there can be a tinge of elitism in an OED citation, as if to say "well, I paid for this, so you should trust my citation more". I have occasionally felt that OED citers ride the fine line between scholarliness and condescension (not always successfully).
M-W strikes me as descriptivist-leaning, but not so loosey-goosey as to be without (enough) credibility as an authority. I think for most people, it is more trustworthy than Wikipedia, for example. I like that M-W's on-line version is free and seemingly equivalent to the print version. It makes it easy for me to transfer whatever level of trust I had with the paper version to the on-line version, as well as easy to cite via URL.
Oh, I am just now noticing that the AHD has its own on-line version as well. I am quite sure this is a relatively recent development (maybe in the last couple of years), since I remember lamenting that M-W seemed to be the only "familiar" dictionary to have a decent (and free) on-line version. (My original attempts to find AHD on-line, years ago, brought me to some other reference site, presumably partnered with AHD's publisher, but definitely not a dedicated on-line replica of the print version like M-W already had for some time. So I stuck with M-W and have become so accustomed to it that I never bothered to look for AHD again until just now.)
A lot of people cite Dictionary.com. I must say I have never warmed to this site. It is not clear to me who its editor(s) or expert panel are. It appears to be a collection of other dictionaries' material, primarily Random House and Collins, but with other stuff thrown in as well. TheFreeDictionary.com is of the same ilk, but seems to use American Heritage and Collins (maybe that's the site I used to get when I looked for the AHD). Anyway, I'm sure these are fine—after all, they are citing perfectly legitimate dictionaries—but to me, they don't have the built-in trust (deserved or not) that comes with carrying the name of an established paper dictionary.
To sum up: OED is a great source if you've got it. M-W is fine for ordinary stuff. For the express purpose of citation, these are probably the best, regardless of whether they have the best actual material, simply because they have the best name recognition and are the most likely to be trusted by the most readers.
I hold Merriam-Webster at the top of the ladder for American English and common usage. I think it is right almost always. A lot of the other dictionary sources used on this site have, in my opinion, either very dated terminology, uncommon (maybe correct) usage, or sometimes plain incorrect. Again this is for American English usage.
I have spent a lot of time in the UK, France, and Australia but not enough to give any sort of helpful opinion on what dictionaries represent other English dialects.