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.