The first I tried was Ubuntu 7.04 but I didn't stick with it and went back to XP. Until I ended up with a hardware setup that wouldn't work on Windows XP (widescreen monitor + Intel graphics driver with no widescreen mode options) but worked perfectly on Ubuntu 9.10. I never truly went back to Windows since.
Tried a few other distros in 2011 then switched to Arch for a couple years, Xubuntu for a couple years, Ubuntu GNOME for 7-8 years, and finally switched to Fedora last year.