Nay, Linux is not an operating system, it's just an OS kernel.

My favourite Linux-based OS is Debian/GNU.

I've used RedHat, and also some of its derivatives. It's a piece of crap.

I respect the SlackWare distro and I would like to play more with it. It's simple, clean, and has a hands-on, bones-exposed, NetBsd-ish approach. (By the way, FreeBSD reminds me of RedHat).

I've used the Hurd for a while, and it's much, MUCH, better than Linux.

User-mode Linux: