It sounds weird but I keep going back and playing Counter Strike Source, purely because it was the first game I really got into (10 years ago.. I was 7)
DoD is still fun, but I never got as many hours in (for reference, I have about 1300 in CS:S and 50 in DoD + DoD:S)
I bought CS:GO before it got as famous/popular as it was, and on release it had that tacky UI that it shared with my xbox 360 (which they stopped updating (yes, CS:GO has an xbox 360 version)).
I bought TF2 back when it used to cost, so I'd consider that an achievement?
Me and my friends used to be really into the COD Zombies series, and I still adore it, I think I have just about every COD with zombies on it lol (I have MW, WaW, MW2, BO, MW3, BO2, AW, Ghosts, IW, BO3, and WW2 on mixed consoles lol) and I have the WW2 season pass, I like playing BO2's zombies then WW2's just to see just how far it's come and honestly, the playstyle feels the same but the graphics have changed a lot lol.