1. Halo
2. Quake, tied with Doom
3. XCOM: Enemy Unknown and XCOM: Enemy Within
Halo, I don't know what to say... This game really has made my childhood. Halo 2 and it's soundtrack and campaign, Halo 3 and it's flipping awesome custom games on Friday and Saturday nights...
Quake and Doom, hard decision... I grew up also playing these games, and man they actually scared the shit out of me, especially Quake 4 and Doom 3... Not scared of them anymore, but this was far far back.
XCOM has always been an awesome series, I've always loved the squad command mechanic, and I just get the feels when my squad dies, or one person of it dies. XCOM is a great game, and further through you'll just feel a connection to your squad, you'll favor them.