Legend of Zelda: Ocarina of Time - One of the first games I ever played, and still play it to this day. I own the 3 copies of this game, 1 N64 2 Gamecube, and I'm extremely excited for the 3DS version and will probably buy it day 1, even though I don't own a 3DS.
Pokémon: Red & Blue Version - Pretty much the same as LoZ, I played this back in it's heyday and still occasionally pick it up and play it, 14 years later. If you are only taking 1 version, choose Blue version.
Fallout 3 - I've put 500 hours into this game, and before Fallout 4 comes out, probably at least 100 more. I loved all the looting and making my character into an unstoppable bad ass.
Mass Effect 2 - The story was engaging, the characters were interesting and likable (The fact that I like and hate characters due to their personalities should be a testament in itself) and the gameplay was fun and enjoyable. There's just something about the Vanguard that makes me want to play over and over again.
Portal - I'm probably in the minority when I say this, but I even enjoyed doing all the advanced challenges. There was just a sense of accomplishment I got from doing them that you don't get from games anymore.
That's my top 5 in order (Wait... That dumbed down Mass Effect 2 over Portal? Controversy!
) if that matters.
