I have the Bronica ETRs, the Mamiya 645 Pro TL, and the Pentax 645n and I'm hard pressed to say which one is best.
The Bronica has wonderful lenses plus the advantage of leaf shutters. I've never had light leakage with any of my Bronica backs (for the GS1, SQ-A or the ETRs).
The Mamiya is cheap (at least in my market) and there is a huge array of available lenses that are very cheap. My favorites are the 80/1.9 and the 35/3.5
Both of these models have the advantage of flexibility. You can attach a grip and AE finder and it works like a modern 35mm SLR. Or you can go old school with the advance crank and WLF.
The Pentax is probably my favorite. Although you lose the modularity and flexibility of exchangeable finders, grips and backs, it is clearly the most modern of the three. The lenses are wonderful (I use only the A series rather than the more expensive FA series), the viewfinder is much brighter than the others, and the imprinting of exposure data on the negatives is a fantastic feature. Personally I wouldn't spend the extra several hundred dollars to get the Nii model because the differences are so minor. But the N is a huge step up from the original 645.
If you want portability and are willing to forego exchangeable lenses the Fuji rangefinders are great too. And very compact! The GS is an "oh so cool" folder! Super compact when not in use. The GA series are more modern in design. All of them have fantastic lenses.
Then there is the Bronica rangefinder that offers exchangeable lenses as well. But I hear that it is prone to mechanical problems.
I guess I haven't answered your question at all, but hope the info is helpful.