Lots of votes for the Yashica Mat, but don't forget there is the plain D and also the 635 which was basically a "D" with the option of a little kit so you could also shoot 35mm. (Bit pointless IMO). I have a 635 myself and its great. There are two lenses out there the Yashikor and the Yashinon. The former a 3 element and the latter a 4 element. Maybe there is a microscopic difference wide open, but the supposedly inferior Yashikor is still a great lens!. Nice ones seem to go for 60-90 here in the UK.

Also I see vote for the Mamiya 645. I have one of these too (GAS) I wanted to try MF in an SLR with interchageable lenses. Ended up with a 645J (the lowest spec with "just" a 1/500 shutter). Paid 150 for an almost mint with 55 & 80mm lenses and metered prism finder off the great auction site, so not expensive.

The mamiya is quite a bit heavier than the Yashica, but is just about neckable to walk out with using a good padded (I have optech) strap.