which would it be? This would be the only video game you could play forever and ever. Before you choose your all time favorite, consider the replay value of it. Your favorite game may be Zelda: OoT but Counter-Strike would keep you busy for longer. If it is for a multiplayer game there would still be all the people online, updates, mods, what not.
Mine would be StarCraft 2 just because its everything StarCraft 1 is/was and more, so I know there is no way I couldn't like it. I've also logged probably well over 1500/2000+ hours on StarCraft 1.