Photographers: Out of these four cameras, which one do you recommend?

1)Nikon D750
2)Canon EOS 6D
3)Pentax K1
4)Sony Alpha 99 II
Which camera would you say does the best with low light photography?
What camera would you recommend the most and why is that so?

The Pentax K-1, because I've shot with Pentax for more than 30 years.

You won't get an unbiased answer anyway, but if you're just starting out, a cheaper camera like the Pentax K-70 will serve you just as well - and allow you to eat while learning to use it properly.

I share Andrew's bias towards Pentax, but you haven't said what you'll be photographing in low light - the Pentax is great for landscapes but I couldn't recommend it for sports or anything that needs rapid autofocussing.

For low light, the answer is a no brainer. The Pentax K-1 is the only one of these cameras to have in-body image stabilization or IBIS. This means that it's the sensor that is doing the stabilizing and not the lens, which is how Canon and Nikon do it on the DSLRs. IBIS provides you with the ability to use any lens and still have up to 5.5 stops of stabilization. If you need to shoot in low light, hand held, then you're going to need a fast lens with an aperture of at least f/2.8. Many people like to use a 50mm f/1.4 or 35mm f/1.4 (providing that the angle of view fits your needs) because of their huge aperture which is 4x larger than f/2.8. However, no one make a stabilized 50mm or 35mm prime. This is why IBIS is such a huge deal for those constantly shooting in low light. Hands down, I would go with a Pentax K-1 over any DSLR from Canon or Nikon for low-light, hand-held work.

If you prefer to go with a Canon or Nikon, I would then suggest that you choose the Nikon Z6 or Z7 mirrorless cameras. The only reason for this is that while both Canon and Nikon put the stabilization system in their DSLR lenses, with the mirrorless system Nikon has finally switched to IBIS. The D750 is a fine camera, but since it doesn't have IBIS, I'd skip it for either a Nikon Z6/Z7 or the Pentax K-1.

Sony is another great option. Right now as far as mirrorless cameras are concerned, Sony is still king of the mirrorless cameras. This goes for their capabilities and performance when making photos or video. Nikon's video is still among the worst of any DSLR. So if you need video, too, then go with a Sony mirrorless like the A7R III, or if you don't need to make large prints, then go with the Sony A7S II. The A7R iii is about 43MP but at the expense of high ISO performance, but you get that high resolution for sharper images and the ability to print very large or crop the heck out of your work. At just 12MP, the Sony A7S ii is specifically designed for low-light photography and is the gold standard for high-ISO performance. But, like I said, it's only 12MP which means that you'll be limited to print up to 11"x14". If you have to print larger than 11"x14", then go with either the Pentax K-1 or the Sony A7R III.

I'd skip the Canon 6D simply because it's an APS-C format camera that does not perform well at high ISO settings, nor does it have IBIS. With the 6D, you'll need an image stabilized (IS) lens like a 24-70 f/2.8 which costs more than $2,400. Go with a Pentax K-1, and you can use a f/4 or f/4-f/5.6 lens which is about 1/4 the cost and a lot lighter, too.

Pentax K-1 is also weather and dust proof and has features specifically designed for astrophotography.
Pentax K-1 also has pixel shift where it takes multiple shots and then blends them together to make one image that is sharper and with far better color and contrast than what's possible with a single image. This is due to the way the pixels are arranged on sensors. There are not an equal number of red/blue/green pixels on sensors that use the Bayer pattern (e.g. Nikon, Canon, Sony, and Pentax). As a result, there are more red pixels than green and this causes a reduction in color accuracy and sharpness. Pixel shift cures this by combining images together to equalize the number of red/blue/green pixels. No other camera company does this other than Pentax.

Not a fan of the Sony Alpha 99 ii - there's been rumor that Sony is going to abandon the A99 family and stick with their hugely popular mirrorless family of cameras and lenses.

For low light, the Pentax or Nikon. For everything else, the Sony.