Best built? Hard question to answer . . . how would you really measure this?
However: Assume that all perils affect all cameras equally and then look at numbers still in service divided by numbers produced? Go on gut feel? Highest average exposures between failures? Chosen by most war correspondents? Able to pound nails, or muggers, and still work? Plain heaviest?
Cameras I have that are still clicking that I have never had to repair:
Leicaflex
Nikon F
Nikon F3HP
Nikon FM2
Pentax Spotmatic
Rolliflex 2.8, 3.5
Cameras that I broke and had to get fixed:
Pentax LX
Nikon F4
Leica R4s
Retina IIIC
Leica M3/M6
Is the top list "better" than the bottom list? No. The bottom list were my favorites by far. Perhaps I broke them _because_ I used them so much. . . they were out and about and subject to the world's perils.
Let me turn the question back on the OP: what do _you_ mean by "best built"? How do you propose to measure this? If I wanted to compare the Nikon F2 and the Leica R4s, how would I know one was better built than the other?
Not trying to be a jerk about this, just curious what you all mean.