The first distro I used would be CentOS, followed closely by Gentoo. CentOS was installed on the computers in the computer lab in college, and Gentoo was on the computers in the library. I think I went to the computer lab first. I'm probably biased against those two now, since every time I was using them I was banging my head against the keyboard trying to get some programming assignment to work, or desperately finishing a paper before midnight. :P
The first I installed and used myself was Ubuntu, which I still use. I just bought a System76 laptop, though, and I'm debating if I'll just go with Pop OS or switch to Debian.