I'm not sure why folks keep talking about the Hassy when the OP is clearly talking about 6x7... Of course the Hassy is hard to beat for what it does. But it doesn't do 6x7; plain and simple.
Each camera has its strengths and weaknesses and most of them have been covered already
RB and RZ are great for studio work; really hard to beat, but bulkier and heavier than most for hand held work.
GS-1 is great as a carrying camera, as far as SLRs are concerned. Light, and very ergonomically pleasing with the speed grip and AE finder attached. Leaf shutters are a big plus too.
Pentax 6x7 and 67 are about half way in between the two mentioned above. Easier to hand hold than Mamiya, but not as easy as the GS-1 and no leaf shutters except a couple specialty lenses. The broom handle grip and focus levers make it better for hand held, but not as sweet as the GS-1.
When I owned all three I tended to use the RB in studio, the GS-1 for hand held work and the Pentax for landscapes.
But the fact is that there are plenty of photographers who use each one of these cameras in the areas they supposedly don't excel and still get fantastic results. It just takes commitment and skill. All three are extremely capable cameras.
If you want to delve into the world of range finders there are a few more options, most of which have some limitations but some great advantages too. Fuji has a long history of great rangefinders in 6x7. The so called "Texas Leicas" are optically very good, but all but the earliest models have fixed lenses. And none have built in meters. The newer GF670 offers both 6x6 and 6x7, is extremely compact (its a folder!) and has a lens that is second to none. But once again... fixed lens. The GF670W is wider but isn't a folder. The Mamiya 7 is one of the few modern 6x7 range finders that has exchangeable lenses. Unfortunately it isn't quite as compact as the Mamiya 6 (6x6) since the lens doesn't collapse and the lens specific external view finders can be bothersome.
In the end I think you could be happy with any of these 6x7 cameras as long as you aren't too concerned about "greener grass" issues. If you go for the Pentax just make sure you get one that has MLU. It's very helpful and only the earliest 6x7 doesn't have it.