-Time for my favourite answer: That depends.
Originally Posted by Eralen
I've shot 6x6 and 6x7 in MF, and personally, I find 6x6 to be the most versatile. (I may have been of another opinion if the alternative was 6x8 or 6x9 - 6x7, to my eyes, anyway - is not rectangular enough to really make a strong case for a composition being shot either vertically or horizontally. Thus, I get distracted when trying to figure out whether I'd get a stronger composition by tilting the camera 90 degrees.
Looking through a square finder (or at a ground glass, or whatever) on the other hand, I'm free to consider whether, in that particular case, I would be best served by cropping to horizontal or vertical afterwards - or go for a square print. The negative will be the same anyway, the camera orientation is the same - there's simply less decisions to make.
So - it can be argued that I find 6x6 to be the most versatile in that it takes one item out of my (easily distractable...) thought process whenever I'm trying to figure out how to shoot something. Your mileage may vary.