Which OS has the steep learning curve and is considered hardest?

• Gentoo ( I have been using it for 3 years now, until I have to switch to Ubuntu for research sake. I love it’s philosophy and I kinda feel even my lifestyle changed after Gentoo. Tried it’s successors, redstar, cosmic mod didn’t liked much.)
• Arch Linux ( when I got into Linux, everyone was like, I use arch btw. So tried it first with gnome, then kde, then i3, then i3 gaps and tui, then used openrc, then used runit. Helped me lot to install Gentoo. But Gentoo transformed me into something else)
• Nix OS ( I was hearing about it since 2022. I wanted to try, and now I am gonna install and use it. I’m planning)

My question is, which among these is considered to be hardest and thus by mastering it, one can master linux to atleast some part? (excluding network management, ofsec, netsec, forensics, etc)