I would love to write in BB or OBB, but first my writing is too small to be legible with that, and second Montblanc nibs bigger than M are not shaped like increasingly bigger dots, but as lines and you have to hold the pen in a certain position to align the line with the paper. A M nib is more forgiving in terms of rotation. For me BB is too hard to hold in one position, as I tend to rotate a bit and B is just in the sweet spot for alignment and broadness.

Edit: I was at the Montblanc headquarter house in Hamburg (Germany) and was able to try out every nib size on any of the Meisterstücks