On the right is a comparison photo of the Nikon Z8 and Nikon D850.
I have owned nine of the Nikon D850. Everytime I buy one, someone comes along and offers me too much money for it, and I sell. Then I miss it. It is one of the highest rated cameras in the world. Outstanding image quality and terrific physical balance regardless of lens attached.
The Nikon D850 was commercially available in September 2017. I had to have one ... I had already owned the Nikon D800, D800e, and D810. Nikon hit a home-run with the D8xx series. The D850 beat them all, and just about every other camera available.
I had a major disappointment in Nikon's first attempt at mirrorless technology. The Nikon 1 series was junk, pure junk. I owned several of these and hate every one of them. Their next effort, the Nikon Z series, started out with some negative reviews. I owned a Nikon Z7. I thought it to be the next generation that followed the D850. It wasn't, not by a long shot. I took about 350 shots with it and got rid of it. My next Z camera was the Z7II. I doubt that I took much more than 20 shots before I traded up to the ultimate Nikon ... the Z9. Way too much camera for me. I kept it for two years, but only took fewer than 1500 shots with it. It was an intimidating camera – size and weight made it awkward. I sold it and got my last D850.
A friend of mine is an agent and occasionally gets cameras to sell. He offered me a Nikon Z8 and I jumped at the chance. The Nikon Z8 is highly regarded and positioned as the replacement for the D850. I haven't had it long, but I can say it is a strong competitor as a replacement for the D850, especially going forward.
I need to point out that it is a close competition, with the Nikon D850 winning out in dynamic range. Actually, the D850 (14.8 EV) has better dynamic range than the Z9 (14.4 EV) and the Z8 (14.2 EV) – and it also wins out in low light shooting: D850 (ISO 2660), Z9 (ISO 2451, and Z8 (ISO 2548). DXOMark ranks the D850 at 100 and both the Z9 and Z8 at 98.
At this point, I have two favorites: the D850 and the Z8. I plan on keeping both (but, that may change and usually does).
As for lens, it is impossible to rank any one lens as a favorite. I classify lenses in two categories (for me). The first is walk-around, the second is purpose-use.
My favorite for walk-around: for the D850, the clear winner is the F mount 28-300 lens. I wouldn't shoot an event with it, but as a walk-around lens, it is perfect. For the Z8, the clear winner is the Z mount 24-200 S lens. There is a Nikon Z lens with greater reach (the 28-400), but it is not an S lens, and not as sharp. I don't consider the reach difference to be that much of an issue (for me).
My favorite for purpose (usually birding): for the D850 I use the F mount Tamron 100-400 lens. I also have a Sigma 100-400, but I find the Tamron a bit faster in AF, and the package includes a tripod Arca-Swiss foot (the Sigma does not). For the Z8, the clear winner is the Z mount 100-400 S lens. A bit heavy and large, but super sharp, super fast.
What's your favorite, and why?