My favorite medium format camera that I have owned is my Hasselblad 500cm but for portraiture I favored the RZ67 that I used to own. With it's bellows focussing you can focus really close and Mamiya also offers extension tubes for it. I realize it's not square like you asked.

The Rollie SL66 is a very fine camera. It also has bellows focussing like the Mamiya RZ and is square format like the Hasselblad. It is also expensive like Hasselblad. A very good friend of mine has one and loves it. The only problem is that it is getting harder and harder to find parts for these cameras or so my friend tells me. It would be something to look into before purchasing one.