I went through this question about 2 years ago when I got into MF. I looked at them all and this is conclusion I came to:

I ruled out 645 formats because it is not a huge format for th extra bulk of the camera over 35mm

If I want to shoot in the same format as 645 with a 66 camera I can frame and crop afterwards.

I looked at the various 6x6 choices and went for the Hasselblad 500C/M - it cost 525 complete with 80mm Planar and film back etc. in nice condition, from a dealer (MW Classic cameras, in London). I figured I could have had a Bronica for a lot less money, but the sheer quality and rugged build of the Hasselblad, along with those amazing Zeiss lenses made the choice too easy. Plus the sheer range of accessories is almost endless - it has to be the ultimate MF System. The nice thing is that over time you can gradually build up your kit. I now have three film backs, the 250mm Sonnar, lens hood, prism, side handle - been eyeing up the Schneider 140-280 Zoom though!

Matt