Half-Life2 - Ep1 - Ep2:
Probibly the best PC game i have ever played, you will not be disapointed. The story is great and you don't really need to have really played HL1. If you enjoy playing mods you will notice that allot of mods use HL2. It is a very easy engine to modify using the SDK
This game is very special, different and chalenging.
Cons: thsi game is very short and gets boring but there are a few mods just to make it more challenging.
Team Fortress2: This is one of the best multiplayer games i have ever played, its fun and the game changes all the time. There are now WAY much more stuff you can unlock since the first release of this game.