Truly an impossible question to answer!

This might be a good time to jump in and mention that a good quartz watch is more accurate than any mechanical watch in the world --a fact which some people seem to forget.
So that also complicates what "the best" means....

For "best in the world" I think Rolex probably has the best and most sophisticated in-house manufacturing operation. I don't like the design of their watches much though...
Patek and Vacheron seem to command the most respect but I find many of their watches too conservative. What I love about vintage Heuers is that they are interesting and well designed, but they aren't "weird" or "showy". To me that is what makes them the most desirable watches.

: Hard to know precisely what measure(s) to take for "the
: best" but Patek Philippe and Vacheron Constantin are often
: considered to be at or near the pinnacle, with a few houses like
: Audemars Piguet not far behind.

: Other, more recent brands like Richard Mille might like to stake
: their claim to be there too but for me they need a few more
: decades of heritage and quality to be up there with the above
: and not mere arrivistes. Some of the German brands like A.Lange
: & Söhne have great quality too, but that brand in
: particular is risen from the ashes of an earlier entity so again
: needs time to prove itself.

: Again, it's not like quality is entirely consistent across the
: ranges either - the most expensive Pateks cost 100s of times the
: "cheapest" and are sold in handfuls each year because
: of the manual work and hand-finishing involved. How much you pay
: for additional quality and how much for exclusivity is open to
: interpretation.