It has to be an old black M2 that I bought new in 1964 - it is on its third top-plate and second shutter. Paired with a 35 - occasionally a 40f1.4. Lenses tend to change more than cameras.
It is a camera that I can operate in my sleep (and some shots taken with it - looks like I have!).
It would be interesting to try to analyze cameras as to why they become favorites. Smoothness, brightness of finder, the variations of film advance levers action - minute wear on levers and dials to conform to fingers etc.