Why not CS:GO?
I devoted a significant portion of my preteen years to Counter Strike. When I first began playing, it was a beta and still had Escape missions (which should still be in the game, IMO). On my 56k modem.
Devoted hours, days, months and years, and still I'm fucking terrible at the game. Had to give it up, some things just cannot be.
But seriously, why Source and not GO? I remember sticking to 1.6 because I disliked Source so much, while I felt that GO was awesome during the brief time I spent on it.
Plus, it still receives developer support, which is a must in gaming today (unless you are a Smash: Melee player and your gods have abandoned you).