This. "Best" is not the same thing as "most expensive." They're pretty much equally good cameras in slightly different ways. 6D has a better sensor, 5D has better focusing, FPS, sports stuff.
Considering that the Mark III costs over twice as much though, I don't think there's ever a reason for anybody to spend that much more for a slightly different flavor, unless you primarily shoot sports all the time (in which case, are you a professional? If so, maybe consider a 1DX instead?). If not, get the 6D for the massive advantage of cost.
If on the other hand you mostly shoot low light concerts and such, then get the 6D no matter what. Even if you have 8,000,000 dollars to blow on cameras (and think the 1DX is too expensive), the 6D will still be a better performer if that's what you shoot.