Counter-Strike: Source was actually my very first Steam game, and my first experience with Steam as well. I had bought a physical copy of it (when that was still a thing) and remember having to install Steam in order to play, and thinking to myself "What is this crap?" Little did I know, that crap was one of the best things ever.
I took a long break from Counter-Strike after I became a Call of Duty fan, but just a couple of days ago I picked up Global Offensive during the Steam sale. I really like it and have been playing every day to try and bring my skills back up. Super fun and competitive game.