Another vote here for the Pentax 67, though it is a fairly heavy camera. It's actually quite well balanced, but...still...I would never consider using it handheld. I know other photogs who do, but the awesome mirror slap really degrades your image! If you go the P67 route, I'd really recommend the P67II version if you can afford it and you typically shoot transparency film. The metering on this model is quite accurate. If you shoot mostly B&W and/or are good with a handheld spot meter (my preferred way to work), then save yourself a few bucks and go with the older version just before the II.
The RB67's are good cameras, too, but also heavy and boxy. These babies can't be handheld, IMO...maybe the RZ67. I have a Hassie 500C/M which is much smaller than the RB, but due to its boxy nature even this camera is hard to handhold. All that said, I pretty much use the Hassie and the P67 interchangeably almost always on a tripod. Something else to think about if you go the P67 route...you'll need a sturdy tripod for best results. For lighter weight yet solid performance I'd recommend a good carbon fiber tripod. I use a Dutchhill P900 which holds me P67 with my longest lens (200mm) solid as a rock!
Good luck with your decision.