Samsung Fold 5 (but I don't care for the form factor)
Pixel 8 Pro
S23 Ultra
I really like the Pixel software so if I knew for sure the 8 Pro had better signal strength that would be my preferred choice. As is I'll probably end up with a Samsung.
Having the second two phones you listed I can say that either would be great.
Unfortunately Google didn't put the Tensor G3 in the fold or Tablet which I feel were big mistakes. I still think the Google fold is the best foldable in my opinion but the Tensor G2 certainly holds it back a lot. So I would probably wait until next year for the Fold 2 with Tensor G3. But you said foldable is not your preference.
That leaves the s23 ultra and the Pixel 8 Pro.
I think both phones are a close tie to the best phone of the year. If I had to call it for just one phone it would really be hard. The s23 ultra has a bigger screen which I like the size and square format BUT I hate curved displays. The s23 ultra has a smaller curve but until I used the Pixel 8 Pro I forgot how much a difference a flat screen makes. It is also just a better screen overall than the S23 ultra. Although it's close and the s23 ultra is bigger.
I prefer the square design to the s23 ultra but it has some trade offs in comfort. The Pixel 8 pro is the most comfortable to hold large phone I have had to date. The way it fits in my hand, the thicker side rails and more rounded corners just make the phone easy to hold. It weighs less as well yet it does just about 90% of what the S23 ultra can do hardware wise and more in software.
The s23 ultra does seem just a little faster in some tasks day to day. The battery life is better on the s23 ultra. You have s pen on the ultra. The software on the s23 ultra just seems to have less bugs or maybe not even a bug but glitches. Both are very stable overall but Samsung has a slight advantage here.
I honestly don't see a difference in terms of build quality. Maybe the Samsung has better seams and armor aluminum which is nice but overall unless you look really close to see any difference both look very good.
In terms of cell reception the Pixel 8 Pro is greatly improved. Both better reception, less heat, and better battery life. It still is no where near as fast in peak download and upload speeds which is different from reception. The Snapdragon modems are much faster but reception is much closer now. In one spot my Pixel had better reception than my s23 in the same spot on a few occasions. However there have been times where the s23 ultra is both faster and better reception. With real life congestion on a network and distance to tower there can be a huge difference between what the network is rated and what you actually get so a lot of times in real life the difference between the Samsung modem and the x75 modem isn't as big but in peak conditions where you have no congestion and great reception the Snapdragon x modems are much faster.
So then we have software and here is the other tough choice. Pixel UI which is slick, intuitive, and uses nice animations with no bloatware (other than Google apps) and fast updates for 7 years. You have a more streamlined interface in Pixel UI with a moderate to high level of customization for the relative light Android skin. I have loved Google's leaner version of Android since the Nexus days. Nothing has really changed. In terms of software I feel that it is the best but nothing is perfect and here comes the caveats.
Pixel UI can be buggy. It can be selectively buggy like with one or two apps or system UI buggy. Although I have to say the last year with the Pixel 7 wasn't too bad. But overall when I just know I need something to work I generally like Samsung OneUI for that. However Samsung OneUI can have bugs with apps just like the Pixel, but not system issues generally.
If you have a business and sign documents the s pen can be a real time saver. If you need tight integration with Office or other business software then OneUI is great. If you like to customize everything and tinker with everything then OneUI is great. OneUI also has really nice animations. You can control the camera with the s pen. You have a bigger and boxier screen which gives you more usable space.
In the end the Pixel costs less depending on discounts and promotions for both and is in some ways better than the Samsung but the hardware on the s23ultra is slightly better both in terms of build quality, processor, ram, and faster storage speed. So you have a lot to consider.
As you can see I really like both phones in different ways.
In terms of camera I generally prefer the Pixel shots both in color accuracy and not being blurred. I do prefer the 10x zoom on the Samsung but other than that one use case the Pixel camera is hands down the best.
Hope all of that helps.
Oh and you should really consider the s24 ultra or other s24. They are looking to be really interesting BUT I am concerned about cost with Titanium and Snapdragon 8 gen 3 and the new AI it may add to the already insanely high cost of the ultra. But if the cost stays the same you may want to wait. Pre order should happen in a couple months I would guess.