I have owned the Pentax 645nll, the Bronica Etrsi and a Mamiya RZ 6x7.
The Pentax had a huge viewfinder and with the matrix metering, autofocus and electronic film advance it was extremely easy to use. I also owned the Pentax 645n without mirror lock-up. The mirror is so well dampened that the mirror lock-up is unnecessary.
The Bronica is stripped down compared to the Pentax but it uses leaf shutter lenses which is a advantage for portraiture with strobes because you can flash sync at any speed.
I have not used the Mamiya 645 but I did own the 6x7 RZ67 which was definitely a quality camera.
Quality wise, you really can't go wrong with any of these cameras. You just need to find the one that fits your needs.