Exherbo's fresh on my mind, so it springs to mind first.

But I doubt it's the hardest.

Could perceive GuixSD as harder (~ I swear, even as a bedrocklinux user (with it's longer file paths and other complexities and extra concepts to grok), I'll never get used to the GuixSD/NixOS confounding noisy style of file paths, with those fat hashes in the middle).

And if what's harder, is changes to the file system, different to the conventional norms, then maybe GoboLinux is harder. Nice as the idea is, I never managed to really daily drive Gobo for any meaningful duration. I at least managed to stick with GuixSD for months.

Distros like CRUX, and also KISS and Carbs, spring to mind. It's a different kind of hard again.

But for sure not the hardest. There will be some even more obscure distro I'm forgetting, because it was too hard.

Do distros that just failed to install for some reason, count as harder, in my experience, since they were just a hard wall blocking getting further? Because some distros I did not even figure out how to download, so they'd be even harder?

Ah, but, the question is indeed "tried", not "used".

I tried NixOS, then gave up when I realised how much had to be learned for its package management language that would not be a transferable skill learned (~ and it was then I decided to run GuixSD, and learn guile, lol, I know no guile from a few months in GuixSD years ago). Maybe that's more a case of tried to try.

And LFS's another "tried to try" (several times even). But always giving up before can really say "tried".

Joborun, might be one of the more obscure examples, that had some hardness to it, that I'd actually tried to ride along with... but that was more a (for want of a different description) a penance amidst familiar comforts. [Edit: and joborun kinda invited [/insisted] more attentiveness, so that, in a way, made it easier to be attentive enough to use something arch-based more properly (e.g. not using AUR blindly)]

I dabbled at making my own (~ respin + scripts + things + grander ambitions). That was pretty hard. Though, the hardest part, was the cat herding community aspect. Not running it.

I tried doing hardened gentoo... that hardest?

I've done decibel linux many times since around 2008 (though it was called gentoo studio back then iirc). If one were to partake of decibel without first learning at least a little gentoo... that's hard. And I only did my first stage3 gentoo install in 2010. [Edit: Oh, and last time I checked, the site was down, and couldn't get the stages from the wayback machine... so that's even harder to try decibellinux now. ~ hoping the guy's okay. ~ which reminds, XeroLinux, another distro I've tried, that's hard in a different way, emotionally, concerned for the developers wellbeing.]

And so I don't think I'm going to include bedrock as a candidate for the hardest... because by the time one arrives at a need for bedrock, one is already comfortably familiar with (likely) many distros, and the amount learned already is still more than the sizeable amount of learning new things required for bedrock, and so, like other distros [though, confoundingly, every distro example coming to mind are also each in their own way exceptions to this rule], that just take a bit of time diligently attentively learning and following the documentation, it's not hard, especially from the likely entry point.

So perhaps I just never really pushed harder... if one were to jump sooner to something harder... then it'd be harder. But the things that would seem harder to the novice, are not hard once you arrive there with the knowledge that brought you there.

Perhaps the hardest, might have been the first, simply because as a windows refugee, it was like going from 0 to 1, the initial infinite step change, where everything after that is easier.

... So wait, am I going to say SuSe is the hardest distro I've tried? lol.

Yeah. Final answer. Suse is the hardest distro I've tried. Simply because it was my first.