Either Deus Ex, Half Life 2, Final Fantasy 8, or Dota 2. And probably Telltale's The Walking Dead as well now.
It's pretty much impossible to pick just 1 of them, there's so many games that I love but those are the ones that I love and play the most. I replay Deus Ex, HL2 and FF8 pretty much yearly, especially now that they're all on Steam and so easily installed again. Dota 2 has captured me like no other game has with the amount of depth and variety there is to what on the surface looks to be a simple game. And The Walking Dead is just amazing. Some other games might have better writing, but no game has made me care so much about fictional characters like it has. There's nothing special or magical about Lee, but he's definitely one of the most memorable characters of any game imo.
If I had to pick 1... I'd probably pick Half Life, since that's the game that made me decide I wanted to do games for a living, not just a hobby.