I have a number of medium format rangefinders, although the 645 I have is one of the Fuji giant point-n-shoots. I've found the Moskva in particular and the older folders in general nearly impossible to handhold steadily, although I have come to the conclusion that actual physical vibration was being magnified by the FX-2 I was developing it in and that the camera isn't quite as bad as it seemed to me.
The one I'm the most happy with is the big, old, interchangable lens Fujis. I have a GM670 and a GL690 and three lenses for the both of them. They're big and heavy and wonderful, but they're also 30 years old and every single component of that system has required reconditioning.
The Bronica RFs get a lot of raves, although I have no personal experience with them; one caution, the long lens options are as scarce as hen's teeth these days. Mamiya's 6 and 7 series rangefinders are also well regarded, and I believe it is still possible to buy a 7 in new condition--however, the cost of my entire Fuji system and reconditioning all five components of it is less than you'd have to pay for a single Mamiya 7 body. The 6 may be more reasonable.
In general, I prefer RFs or TLRs in medium format for walking around; while my SLR is a dream come true once I put it on the tripod, it's awkward for handheld work. My work demands a longer than normal lens from time to time, as well as a medium wide, so the 65/100/180 I have on the Fujis, and the 50/75/135 I have on the SLRs works wonderfully. Because of the prices the long lenses on the Bronica RF are going for, I don't regard it as useful for me.