For the 645 Pro, I like the focus assist lever on those lenses it fits on, because I can use that with my right hand.
I am going to say Mamiya 7, but that's because it's the only non-interchangeble back camera I own in Medium Format. I can load it faster than I can load my Leica and can do it with minimum fuss, standing, on the go, etc...
A variation on the subject. What MF camera is easiest to load if you are standing and have no place to temporarily set down camera parts?
The Kodak Medalist.
You can simply hold the camera vertically, flip open the back so it hangs down, pop in the new roll, insert the tongue into the spool and turn the knob. If you try this with Rolleis and many other cameras you run the risk of dropping the camera.
I also thought that the pentax 6x7 was fast and easy to reload.
My rolleicord sometimes gives me problems getting the take up spool seated correctly.
I confess I haven't read every post in this 4 page thread, but with regard to all the questions about needing to set the camera down while reloading while shooting handheld, all you need is a neck strap. I think every MF camera takes a little getting used to, but that's part of the fun. Certainly cameras with removable film backs take a little more work, but then you have the advantage of being able to use multiple backs and preload them at home. I was glad to see cjbecker's favorable comments about the P67. It has a reputation for being hard to load, but I've never found that to be true. You just have to spin the spool a bit until it is properly seated.
I have 4 MF cameras of different makes and, believe it or not, my pre-WW2 Baby Bessa is the easiest and fastest to load. Voigtlaender's design used swinging half-cylinder holders for the film spools that line up the feeding roll perfectly with the film-advance sprocket. Works like a dream.
Otherwise not up to Hasselblad or Mamiya standards, however!