My favorite os? OS/2 Warp 4.5 (Merlin). I have it on a P2 in my basement, and in a Vobox VM.
It's awesome. To think we might not have had to put up with Windoze so much if OS/2 had been marketed properly...
Anyway, I'm primarily a Linux man these days, but I distro-hop like a mofo. So I'll list the regulars by frequency of use: Linux Mint, Linux Mint Debian (generally for my "standard-use" machines); Debian Squeeze for servers, Arch or Gentoo for my occasional specialty or pleasure use, Ubuntu for my family, OpenSUSE 11.4 with rolling updates for my design machine and poor man's render cluster, as well as programming. I have a lot of these under Xen on my main work box.
The general-use OS I use is Mint on my desktop (which I might change to Fedora 15, since it's the first Fedora I *really* like since 9.) Laptop has Ubuntu 11.04 currently, though I'm going to give Sabayon a crack. Both dual-boot windows (for games, video, and photo work only.) The desktop also has a Xen hypervisor partition (for all the other OSes.) I might go with KVM later on.