Depends on what you're looking for.
I cannot recommend NixOS enough, it's such a good distribution but on the other hand it's quite tough to learn as it deviates a lot on how distributions do things. It still uses a standard stack (glibc, systemd, GNU tools and all) but the nix tools which include the package manager are totally different from what other distributions offer. It's very solid, yet flexible. It offers a lot of packages by default. I've switched my machines to it because of the advantages.
Arch is great as a rolling release distribution with solid repositories (lots of packages and quite up to date) and it's very close to upstream with a more traditional approach to the distribution tools. In fact there aren't really any apart from the package manager by default. I feel this is one of the most comfortable distributions if you want to learn how a classic Linux system is structured. I ran Arch for about 15 years and didn't really have anything to complain about and I learned more about Linux there than with Ubuntu and Debian.
Please note that neither of these are what one would consider beginner-friendly distributions.