For myself, the format is often chosen to fit the subject. I have long used the 6x6, 6x7. 6x9 format, but to say I preferred one of them the over another
I couldn't do.

When packing a 6x6 I compose full frame, as I do with all formats. If there is a question in my mind that perhaps some one else (photo editor etc) might prefer a horizontal or vertical image I manage the image in such a way it can be cropped. Actually unless shooting to a script, I allow myself to make the image any way I feel it or see fit. I do a lot of square images, but in no way feel that it is a favorite. I do not set out with a Haselblad in hand looking or searching for subject matter to fit into a square.

Uhmmm, upon thinking more about it perhaps I do lean a bit toward 6x7. I don't however lean enough towards it to call it a favorite.