While the landscape of 'atomic' distros may look thriving, we can distill the amount of mature projects to just NixOS, Guix System and Fedora Atomic (and their derivatives).

Most of the other projects found e.g. here are unfortunately not worth your time in most cases.

As for which one to go with, that definitely depends on what you desire out of it (in alphabetical order):

• Do you want something that's relatively easy to start out with? Then go with Fedora Atomic (or a derivative^[The images found under the umbrella of uBlue (which includes Bluefin) are even easier.]). From my personal testing, this one also seems to be the friendliest in terms of taking away many footguns.
• If two of the following strike a chord with ya, then you should heavily consider Guix Systems:
  • Enjoying/liking^[Or, at least finding it intriguing.] Lisp
  • Being passionate about F(L)OSS
  • Enjoying NixOS as a model, but being discontent with something^[Including, but not limited to, Documentation; DSL vs actual programming language; project governance.] that happens to be (better) (re)solved on Guix Systems
• NixOS is the OG^[To be clear, it's almost as old as Ubuntu.]. As such, there's a lot of cruft. But within that mess, somehow, ironically, it holds the potential for setting up the cleanest OS. As you can guess, without a shred of doubt, it is the most mature and most powerful out of the bunch. So, if you've got exotic places you'd like to go with your distro, then -in a lot of cases- this is the only viable destination.