Damn, how long did you stick with Knoppix?
I had two firsts—I messed around with Ubuntu around high school or so, but I don't count that because I was only curious and had no intention to actually try and use it for any decent stretch of time.
Second, which I consider the "true first", was Fedora, and man was it dope. It's the distro that made me realize Linux is a lot more accessible than I had thought.