Mine was DamnSmallLinux and Slax. I wasn't allowed to install Linux on my family's computer of course, so live CD's were my go-to. Though once I scrounged up enough spare parts from family friends to build a PC, I then got to learn how to force Slax to "install" to a hard drive, which was quite a challenge for me, as it really didn't want to be run off non-live-install media haha
My first distro was Ubuntu 8.04 Server. Kind of funny because I was in high school back then and decided to install Ubuntu on my machine, coming from the Windows world, I thought that it would be cool to run the server version, thinking it would be like Windows Server 2008 (with a desktop). After installing I was shocked to find out that I had no desktop to work with, spent the next 3 days figuring out how to connect my machine to the internet and then eventually downloaded Gnome. Now almost 15 years later I still use Ubuntu as my daily driver, typing this on 22.04