Hmm, it would have to be the old Nikon F. I have a Hexar, but I can actually shoot a manual focus SLR more quickly than the Hexar focuses. Also, the Hexar has problem focusing through windows and such. The old Nikons are easy to use, rock solid reliable, and require no batteries. They also can go decades without requiring any type of service or repair. Lastly, they can be used with dozens (hundreds?) of different lenses.
I like Leica M cameras, but they require regular servicing to keep them on top shape, and more than once I have managed to bump an M hard enough to knock the rangefinder out of adjustment.
If this thread were about the best film camera of any format, rather than being limited to 35mm, I would say that the Rolleiflex TLR is my choice.