- OED is the best ever, and you probably have free access to it somehow (but you may have to look for it).
- You can assess quality systematically by choosing some set of characteristics that are objectively measured. Those characteristics should include:
- number of entries (semantically different definitions) per term (a proxy for nuance)
- length of entries (a proxy for descriptiveness of the definition
- entries on etymology
- variety of spellings, variety of pronunciation
- specification of varieties (register, geography, technical area, etc etc)
- frequency information (how common a word it is)
- You can assess quality quickly by taking a small set of words and comparing their definition in all the dictionaries.
There are some characteristics not mentioned. Part of speech is managed by the entry labels. Accuracy of the actual definitions would take a statistical analysis of the content. This latter is probably the -one- thing you care about and not all that scholarly drivel above. But it is also probably the most subjective of criteria.
These are ways of assessing but I don't they have been done. But by inspection of any dictionary's entries you can quickly compare.
There is a general consensus that the OED, in book or print form, is the best dictionary in English, in the all around quality, accuracy, and amount of content of its entries.
- Its definitions are concise while but not too concise.
- It has subentries for each semantic nuance to a term.
- It has phrases including a term.
- It has the currently accepted etymology.
- It has multiple instances of the term used in the wild, including the first known use.
While other dictionaries may do any one of these, the OED combines all of them.
By authoritative, I think you imply some slightest bit of prescription - if there is a dispute about correctness, which choice should be made. While that is a practical goal for many users of dictionaries, it may incur too much contention.
There is no academy (à la française) of a few highly regarded people of letters behind it (as much as the French Academy is an authority, the French language still changes and adds new words without them. Also the AF does not produce a reference book like a dictionary or grammar). The many people behind the OED include a history of editorial group, a set of scholars writing the definitions, plus hundreds of people who find earlier and earlier instances of first sightings of each word.
The OED is not perfect. It is rife with factual errors, omissions, typos, bad dates, and questionable life choices. That said, any other dictionary is much much rifer. For example, the OED is pretty good at assessing whether a word is more AmE or BrE (using 'primarily' as a marker). But it is (supposedly (I have heard)) not so great at varieties with smaller populations or less media presence (AAE, AusE, IrE, ScotE, and on and on). But really do the other dictionaries come close to that? Also it is supposedly not very good with taboo words or slang. Oh well, Urban Dictionary does excel at that.