That's an interesting question Pete. It is not as simple as it sounds. There are different factors that are interrelated but not necessarily at the same level of quality on all instruments. The ones that come to mind are: 1) Design, 2) Quality of components, 3) Manufacturing detail, and 4) Key Fitting. Three of these factors can be summed up with the term "quality control".
I have worked on more saxophones than other high end woodwinds so I can attempt to address the makes I am familiar with. Saxophonists are fortunate today in that there are many high quality instruments to choose from: Selmer, Yamaha, Yanagisawa, Keilwerth all appear to be well designed, constructed, and have good quality control. As a disclaimer, most repair techs do not see instruments fresh from the factory on their bench unless they work for a music store that "sets up" the instruments before they are sold. Most of my work now is on instruments that have seen better days.
Some of the top saxes from Taiwan are being made every bit as well as the "big four". Cannonball and P. Mauriat are the two I am most familiar with. Mainland China is getting better, and it is just a matter of time before they are competitive with the rest.
To put the question in the proper perspective---if I asked you which is the best put together computer, how would you respond? See, its not that easy. :smile: