Today's photographic challenges call for capturing as much of the scene as possible, as quickly as possible, in very harsh conditions, From capturing the Milky Way to the Northern Lights - Full-Frame sensors provide the best image quality - and give you the best chance for success in post-processing.
(not listed in any particular order)
The Canon 6D is a great full-frame sensor camera - containing about the same megapixels as the Canon 5D III. I shoot with it regularly and I recommend it to shooters who do not need the video aspects of the Canon 5D III/ IV series.
The flagship Canon 5D III is well known for its image quality and professional statue. It's the core of a great system of accessories and amazing lenses which can handle virtually any professional assignment.
The newest addition to the Canon 5D line offers 36% more image resolution - as well as 4K video. If you assignments involve a mix of video and stills - this is a great camera.
I see a lot of the 810 in my workshops and in the field. It's an amazing camera and it does a great job with astro and landscape photography.
The 750 is a great full frame camera. While it does not have the megapixels of the D810, it provides great image quality and is part of the great nikon system of lenses and accessories.
The A7R II is the new kid on the block. It offers an amazing sensor which can handle low light and deliver clean RAW files well into ISO 4000. If low-light is your scenario - then you need to try out the Sony. The drawbacks are that there are limited lenses available for the Sony at this time, but third parties are stepping forward to fill in the gaps. Try it out!