I agree with the comment that there are no super-reliable mechanical medium format SLRs, like the Nikon F2. They all have their faults. The RB67 Pro SD is probably what you should go with. However, the Pentax 6x7 is quite rugged and reliable long-term, despite the dependance on a battery. These cameras are getting very old, and they'll all need service. A classic Rolleiflex TLR or Hasselblad are reliable and serviceable, but 6x6 of course.
I have a Makina 670 (not a SLR). I would not call it reliable, though being mechanical it should be easier to repair. I think the Fuji GW series is generally more reliable and simpler, if you're okay with a fixed-lens rangefinder.
The Mamiya 7, Pentax 67II, and Mamiya RZ67 are all relatively reliable for electronic cameras. One of the biggest problems with many medium format cameras seems to be the winding mechanism - on a RB or RZ, you can just replace the back. Mamiya 7, Bronica RF645, and Makina 67 series are all noted for developing winding issues over time.
If you're willing to consider other, weirder rangefinder options: Mamiya Press series, Koni-Omega Rapid series, Marshal Press, Horseman VH-R, Linhof Technika 670 or Press, etc. were professional cameras. Some have been used to death already, and others are in good shape. They are mostly simple and easy to work on, with no electronics. Personally I would avoid Pentacon Six or other East German/Soviet cameras.
