A professional camera system is all about the system support.
For me the key is good support from camera RAW processor software and good flash support the the major TTL providers for when I do event and wedding photography.
There are not many medium format players left but lets start with the big boys.
Hasselblad H6D-400c: a full frame medium format camera that supports multi shot tech, that will produce a 400 megapixel file from its 100 megapixel sensor. It has 16 bits of colour data and 15 stops of dynamic range. Full support from the major flash manufactures but at over £40k you are paying for the best. If you want the best the H series cameras are hard to beat.
Hasselblad also produce a few cropped sensored cameras, its most famous the X1D 50C, now on version 2.
Phase with there IQ digital backs are at the leading edge of medium format, there XT system camera with a 150 megapixel back, plus backs for legacy cameras such as Hasselblad, and since the purchase of Mamiya there own range of medium format cameras. If your interested in system flashes then like Hasselblad support the Nikon flash standards, Phase support Nikon and Canon flash standards.
Many would say Pentax started the compact SLR type handling autofocus medium format camera. Their 645 system is often forgotten about but with its cropped 50 megapixel sensor is something to consider and for medium format a budget way of getting into things. There TTL flash support is limited but the price is tempting.
Fuji was actually the subcontractor for some of the Hasselblad bodies and lens. In the modern digital age they have been making a name for themselves with their cropped 35mm sensors. Now to partner these they have brought out the for medium format budget GFX range. For just over £3000 you can pick up a body, £10,000 for the 100 megapixels versions.
What would I pick? Budget no option I may go Phase but as I do have a budget it would likely be Hasselblad at £5400 for the X1D II 50C.
For many pressionals we would likely generally use a Canon 5D or Nikon D850 type camera, and for when we need to reel out the big boys, most rental houses carry Hasselblad H and X series cameras.