Simple answer for me - Mamiya RB67 all the way. No hesitation with this question. I have a Pro-S and sorta wish i had bought an SD, but it's not too much different. I *love* the format, though 6x7 is harder for me to get C-41 films developed (like near impossible).
For smaller format, i really like my Minolta SRT202 and stable of excellent Rokkor lenses. I also have a KM 7D dSLR that is an absolute joy to use as well, but that's not an APUG appropriate answer!!!