Linux

Lots of great answers here already so I will only address a couple of things that haven't been mentioned:

Regarding Fedora Silverblue:

• Currently, Fedora Atomic Desktops are in a major shift to accept OCI container images for delivery of packages. This means that the built image becomes one compliant to OCI and that we boot into an OCI container as our system. As OCI images are relatively declarative (not to the extent that NixOS does (yet)), it becomes possible to have a set of config files (most importantly, the so-called Containerfile) in which your system is 'declared'/'configd'. In case you're interested into how this looks/works, consider taking a look at uBlue's startingpoint or if you're more interested in the scope of configuration into Bazzite and/or Bluefin.
• apx is available as a COPR on Fedora Atomic Desktops.
• Nix can be installed on Fedora Atomic Desktops using Determinate Systems' installer.

Regarding Vanilla OS:

• They're also moving to a model that's very close to where Fedora Atomic Desktops is heading towards. So, expect a similar way to config/'declare' your system.

What are your thoughts on the ~~three~~ four distros mentioned above?

It's a question of polish if you'd ask me. With Fedora Atomic Desktops and NixOS being advantageous due to being more established and better funded. I wouldn't write off Vanilla OS yet as they seem to know what they're doing. Though, I wouldn't keep my hopes up for blendOS as its main developer was unaware of which MAC was configured by default on blendOS (spoiler alert: none, at least at the time).

Furthermore, NixOS is literally its own thing and unfortunately infamous for its steep learning curve. If you can afford to learn and conquer NixOS, then NixOS should be the recommendation; unless (like me) you seek SELinux on your systems.

Between Fedora Atomic Desktops and Vanilla OS; Vanilla OS is still in its major rewrite/revamp. The alpha builds are there, but I wouldn't recommend using those on production machines. Fedora Atomic Desktops, on the other hand, has been going strong for a while now and the uBlue-team has even succeeded in making the OCI-stuff accessible for the general (Linux) public. So if you want to switch now and NixOS is/seems too hard; then Fedora Atomic Desktops it is. On that note, I recommend to check out the uBlue project.

Which ones are the most interesting, and for what reasons?

Honestly, all of them are really interesting, but NixOS does the most unique stuff; with only Guix doing something similar within the Linux landscape. To give you a taste of some of the wild stuff found on NixOS; there's the so-called Impermanence module which -to my knowledge- happens to be the closest thing to a usable stateless system we've got; period. Consider reading this excellent blog post in case you're interested to know what this entails.