The first FOSS product I ever used would have been either Firefox or OpenOffice.org, back in ~2010. I also used to like VLC.
The product that got me to go almost exclusively FOSS, however, was Linux Mint. I installed it on an old ThinkPad that my uncle had given me in 2019, and I was immediately impressed that this twelve-year-old notebook with (at the time) 4GB of RAM and an Intel Centrino processor could now easily outperform my brand new HP (which ran Windows).
It was only about a year later when I installed Mint on my HP, followed by my old Acer (which had been on a shelf for the last two years), and most recently my 2007 MacBook (which I keep around because it's the only thing that can operate my scanner).