Get Half-Life. Of any game in the WORLD, its gotten the most awards, and by most avid gamers, is literally thought to be the greatest game in the world.
Portal 2 is also great, but playing any of the Half-Life games is like personally experiencing a piece of video game history.
Zaln, Half-Life is an AMAZING game for PC. I got the first Half-Life when it first came out and litrally didn't leave my house for friends or to be social for about 1-2 months. I then later scratched the disk. :?