Android

I am still loving my Pixel 6 Pro. Other than installing AdGuard, or just change your DNS, I pretty much use it as is. I turned off all the Google home or whatever that thing was to the left. Take a minute and go through your installed apps and turn off notifications for things you don't want or just uninstall. I don't think you could ever convince me to buy a Samsung again.

My only gripe is the size and how the camera sticks out. The battery life is great, I have plenty of space locally, and I can wirelessly charge my earbuds with it if I want.

Any with good hardware as long as you can switch to stable and community-made ROMs.

It is easy to turn off ads on Xiaomi. I am using Poco X3 pro for more than two years and never seen one. Bigger problem with Xiaomi is media selector app and that they disable auto unlock Android feature (you can't unlock your phone automatically when at home or some BT device is detected).

I have the Pixel 7 Pro and there's no ad forcing. Some apps can push ads, but just block them or don't use those apps. I'm using the Nova Launcher though. Idk if that matters.

I currently own a S23+ and had a OnePlus 6T before that and Nexus 5X before that. I wouldn't be bothered by the Samsung bloat. You can uninstall nearly everything. Set the default apps you want to use and hide the rest. You have this configured quickly and then you will never be bothered by it again.

The default Pixel 8 launcher doesnt even allow you to remove the huge Google search widget, which bothers me a lot more. I find One UI pretty vanilla feeling to be honest. Don't notice that much difference from my OnePlus 6T which had a fairly vanilla OS.

I have the Xiaomi 13T Pro, and I haven't noticed any ads. You can buy wallpapers and ringtones, but there are also tons of free ones, and it doesn't pop up advertisements for it except once to show the function is there. The new HyperOS although it looks the same and has similar functionality on the surface as MIUI, is way way better IMO.
Security and game center are gone, 2 of the top annoyances with MIUI IMO. HyperOS feels way snappier too. With MIUI the 13T Pro was slower than my old Motorola to turn pages in an e-book quickly. Now it's super snappy. HyperOS just feels better somehow.
I installed Firefox with uBlock Origin, and chose it as default browser. I also uninstalled "Community Center" I think it was called. An app to use various Xiaomi services.
There are a couple preinstalled apps, I don't recall if Facebook was among them, but they are easy to uninstall, it literally just takes a minute to uninstall.

But maybe Motorola would be more to your liking, they use almost completely vanilla Android, with only an app to configure Motorola specific features. I did get some ads for printing services on my Motorola, but it seemed to be more a Google thing than Motorola. (Haven't seen any such things on my Xiaomi 13T Pro)
And AFAIK you can still remove the search widget from the desktop on Motorola, something that apparently is no longer possible in completely vanilla Android. Reason enough for me to completely avoid Google Pixel phones. Why would they even want to do such a stupid thing?

What ads are you referring to? I gave Galaxy S 23 Ultra and I've never seen any ads in the OS.

The easiest way to get rid of ads it's a DNS level AdBlock, you can simply use the private DNS option of Android(with AdGuard or Mullvad free DNS resolvers), next step use alternative frontends for YouTube(NewPipe x SponsorBlock) and Music like Spotify and YT Music(xManager or ReVanced), there will be some apps what will keep showing ads like Facebook but at least it will be a minor amount.

If you want to be more sophisticated about Android AdBlocks I will suggest you to root your phone, but with these non-root options 90% of people will do pretty fine.

Oneplus

They will hate me but iPhone has BSD/XNU/openSTEP. Of course it may feel like a prison after Android. Either that or a Linux phone with Waydroid