I've pondered those questions for awhile. This article on bbc.com finally motivated me to post on this subject.
http://www.bbc.com/news/magazine-35503084
To illustrate, I participate frequently in the "South" forum on FT, mainly because I live in New Orleans. Frequently, someone will post in the NOLA restaurant thread that "
X" restaurant is the best in New Orleans. Of course, they've only been to the city once and haven't eaten in every one of the 1400+ restaurants in the area. I usually shake my head and think that particular poster must give free rein to hyperbole quite often.
In my opinion, you can say a restaurant is your favorite of the ones that you've tried. Or, the best of the ones you've tried. But, best restaurant in a particular locale, without having eaten in every restaurant? I think not. The same goes for naming the best restaurant or chef in the world.
I'd be interested to read other opinions on the "Best Restaurant or Chef" and, hopefully, we can avoid Omni territory.
Edited to add my other pet peeve: To me, there can only be one "Best" of some specific thing. How can there be the 50 best restaurants or the 10 best chefs? There could be 50 great restaurants or 10 phenomenally fantastic chefs. I guess that when I see "best", that means the pinnacle and there can only be one "best".