I'm only counting distros I actually used for more than a day or two, and yes I'm counting all *nix's, not just Linux...
In order since 1999 (when I discovered Linux):
Redhat 5.x, Redhat 6.x, CalderaLinux (loved the Novell Client built in), Various Fedora/FedoraCore's, Solaris 10, Solaris Express, OpenSolaris, Ubuntu 9.04, Ubuntu 10.04, Debian Etch, Ubuntu 11.10, Ubuntu 12.04, Debian Squeeze.
There were a few years (2004-2009) that I really liked and used Solaris quite a bit, but it lost me when the whole Oracle purchase went through. Fedora lost me when Ubuntu came along, and likewise, Ubuntu has recently started losing me to Debian. I like to think Debian's got the right mix of what I'm looking for (I'm posting this using Squeeze) and don't plan to leave anytime soon! Slow is fine with me, I want it to WORK and Debian does a good job of that!