Top 3 (ish)
Excited to see my top pick hasn't been mentioned
1) Contra (Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start) The birth (one of the first) of the console "cheat"
2) Gran Turismo - I agree 110% with 'N00bvin' (notice the lack of regard for numbers) This game raised the bar and set the standard for physics and gameplay when it comes to racing simulators.
3) I'm gonna go on a limb and say (I'm gonna date myself here) Zork (the original). If you know of the game you will understand why I am going on a limb