I've used Ubuntu, Redhat, Fedora, LFS, Gentoo, Mint, Debian, Mandrake, open SUSE, OpenWRT and Slackware.
I've always pretty much been a Slackware user, however, and was ultimately unhappy with any of the other Linux distro's I've tried.
Linux From Scratch (LFS) = too much work
Gentoo = too much work
Debian = gets in my way
Mint = gets in my way
Mandrake = too buggy
Open SUSE = gets in my way
Redhat = gets in my way
Fedora = gets in my way
Arch (never used) = too autistic
Slackware has what I consider the best Linux distro maintainer out there, stays out of my way, is rock stable, and very simple to use.
OpenWRT is a great distro for embedded systems and I use it on my routers.
Other thoughts: if OpenBSD had better hardware support I'd probably switch to that -- super simple easy to use design and very stable.
I don't like SystemD I tried it on all the distro's that include it now, since every once in a while I retry them. I've tried every distro in that list from the time they first came out (with the exceptions of Debian and Slackware which I tried a little after they first came out) and retried them (the then current versions) every once in a while over the years.