Ohh man, is it possible to have one favorite game?
Grew up playing C&C and Might and Magic VII. Still play them both at least yearly.
Every Nintendo and Super Nintendo game ever!
Spent many years playing CS:S (Counter Strike: Source), where I was the leader of a clan, created my own maps (Hammer SDK), admin'd servers, directed/organised comps, etc.
My gaming name (The7thLegion) originates from the '7th Legion' game, an RTS from 1997 that I absolutely loved.
Current games include: Dota 2, Destiny (PS4), Diablo III, WoW, and CS:GO.