Interchangeble lenses ?

6x4,5, 6x6 or 6x7 ?

The cheaper ones:

6x6: the Rollei's: the Cord's are good, not so expensive and repairable, the Flex's even better, but more expensive
Mamiya C 220/330 if you want more than one lens on your camera

6x7: the beast, the RB67, or a bit lighter and more expensive the Pentax 6x7