Why not start out with justifying your own choice, instead of just asking people to justify theirs?
Anyway, as for me, I'll choose the Half-Life series. The first set up the series with a bang. Puzzles, smart AI, fun weapons, decent graphics and for me a good setup in the story department for setting up the rest of series.
The second game, my favourite of all time is just fantastic. Great atmopshere, great characters an awesome universe and of course G-Man. I recently played through it again and it was just as fun as the first time I played it. Some say the gameplay is meh, but I like it, I like the variation between driving, shooting and solving puzzles. The episodes just deliver more of that.
So there you have it, my reason as to why the Half-Life series is MY favourite of them all.