For me, perhaps the most important thing, once all the basics are covered (accurate shutter, proper collimation, reliability, etc.), is an excellent viewfinder / focusing screen.
That will allow you to easily and accurately focus even in difficult light conditions or with "difficult" lenses.
For that, the Leicaflexes you mentioned are unbeatable (despite what Olympus OM-4 fans claim).
Other good things: Lenses!!! Also, good ergonomics, spot metering, DOF preview, well-dampened shutter and mirror (at least for 35mm SLRs, a well-dampened mirror makes MLU unnecessary IMHO).
Of the cameras you mentioned, my first choice would be one of the Leicaflexes, then the Contaxes for their Zeiss lenses (not really for the bodies), then all the rest, which are very good except... the Leica R3, which I would avoid (if you must get an electronic Leica R, find a working R4, R5 or R7. The R8 & R9 are wonderful cameras too).
The Pentax 6x7 is a lovely camera, it just takes some guts and energy to carry it around... ;-)
If you get one, make sure it has MLU (in that case, MLU can make a difference!), though its mirror slap isn't as bad as legend has it, since most of it is the second shutter curtain closing and the mirror returning.
If you decide for one of the Leicaflexes, ask and I'd be glad to give the pros and cons of the SL vs. the SL2...