Framework. I’ve run Debian, Fedora and for a while now NixOS, all of which have worked flawlessly.
I did have to replace the heatsink/fan part on mine because the fan bearing started clicking, but I’m sure that was just a first generation product issue (I was one of the first batches). I was glad to be able to do the replacement myself at relatively low cost and the process couldn’t have been easier (took about 30 minutes).
My previous machine was a 2013-ish ThinkPad X series and the Framework absolutely blows it out of the water. I’m looking forward to upgrading mine to a Ryzen motherboard sometime in the not so distant future.
I have used all three! I started with Server then went to CoreOS running Kubernetes and settled on NixOS which I have been very happy with for about a year now. I run about 25-30 services all using built in modules.
Regarding security, if you are using well crafted modules on NixOS, there should be good systemd hardening in place. That being said there is no reason you can’t just use containers on NixOS.
I also find deploying NixOS far superior to butane/ignition used by CoreOS/Fedora. I use nixos-anywhere and can deploy my entire server in a few minutes without manual intervention.