Definately 6x9, with 6x7 being a close favorite. I don't like 6x6 because I just can't get used to square negatives/images, and 645 is just too small. I've never had a chance two try 6x12 or 6x17, I don't think the number of frames or lack thereof I'd get on a roll would bother me as much as that fact that thats just too wide for my taste. 6x9 is where it's at, though the camera choices aren't all that great unless you like folders or plastic bodied point and shoots like the agfa clack. The Zeiss Mess Ikonta's and Super Ikonta's are nice but it's hard to find a good working one, and when you do they're usually kind of expensive. I now have a Medalist and I'm going to try it out tonight, I believe that it's going to be my new favorite medium format camera. It can't be my new favorite camera because that would have to be the Crown Graphic.