Camera bags are very individual choices, and even then, most of us seem to have several. I haven't counted mine lately, but I guess there are between 5 and 10 of them. For medium format, I used the TT Airport Addicted II for years. It's basically a huge brick with shoulder straps and lacks any features other than raw space and a big pile of dividers. I bought it for two reasons:
- It rooms four small studio strobes plus lots of accessories.
- It rooms the GX680 plus a couple of lenses, an extra film holder and the AE finder.
For anything else, it's pretty useless.
My favourite bag that can room a MF camera is the Lowepro ProTactic 450 AW (I also have the 350 which is suitable for m43). It takes my Mamiya 645 and as many lenses as I care to carry and a couple of film holders, has 3 openings to the main compartment in addition to the fully opening back and you can run over it with a truck. The only real disadvantage is that there isn't any room for non-photography gear (a warm jacket, extra clothing etc.), so there are limitations when travelling.
Because of those limitations, I recently bought the Lowepro Whistler 450. I considered many alternatives, and the Whistler was the conservative choice. It's not very flexible, but the layout suits me and there's an expandable front pocket that is suitable for extra clothing etc. It also has a top compartment that rooms one m43 camera with lens plus my rather large headphones. One disadvantage is that the waist strap can't be removed. I rarely do long treks, so actually consider cutting it off. The main alternative was the TT Backlight 45L.
All of the above backpacks have very rigid, flat surfaces towards my back. While that isn't the most ergonomic solution, I live and travel in a tropical environment, and don't want more contact with my bag than necessary, since I will sweat and want as much sweat as possible to evaporate even when I carry the bag. No, I don't believe in fancy air channel solutions. Not anymore. Had I been living in a colder climate, I might have chosen differently, but I'll probably never find out.
The Whistler can take the GX680 in a pinch, but I will probably buy a big shoulder bag for that. There are many bags for large video rigs that are suitable.