either..
Minecraft, Mass Effect or Fallout 3
It almost pains me not to list an assassins creed game but the difficulty is pathetic now.
The three contenders chosen all push innovation to an extent, never before playing Fallout 3 have i experience a karma system that makes even a slight bit of sense and a useful custom build of skills all of which you could need but you decide (like skill trees in an rpg but it feels as if you need them).
I have listed Minecraft as it really is amazing. Building a world of your own from scratch is awesome and the lack of tutorial for me really made the game work, its the best way to learn.
Minecraft doesnt need a tutorial is you're supposed to be in control of everything to an extent, even if the game has a nasty habbit of pushing you into a bunker for the first one or two days it has achieved greatness especially being a beta.
Mass Effect in my head is constantly changing from good to bad, the blend of customisation in guns and armour was good for me and i did playthrough more than thrice aided by the great biotics system which synergises nicely with gunplay but it feels so linear, the story has dialogue options sure but it doesnt achieve anything.
On second thought, Ill choose Fallout 3. Growing up in the Vault isnt a kind of introduction you experience everyday and i do so love my calm introductions to game with action and horror elements such as Half-Life and Blue Shift.