This is an amazing set of games. If you don't play these, you are not a real gamer.
Half-Life 2
(Before I begin I just wanted to say that I haven't played the first Half-Life yet. I plan to, though.)
I was very satisfied with this game. I love the gravity gun and of course, the crowbar.
The designs and story for this game are AMAZING to say the least. The combine were very well thought out and Gordon is a f**king badass. eh hits aleins with his crowbar and doesn't afraid of anything.
The gameplay is very fun and I love a nice shot to a headcrab zombie.
Overall, I would give this a 9.9/10. I just hope Episode 3 comes out soon.
Team Fortress 2
I have spent almost 400 hours on this game and every second was great.
Team Fortress 2 (Or TF2), is a multiplayer only game that is very balanced, fun and often updated with new content and bug fixes.
There is a vast library of weapons and even accessories to unlock and trade to other people. (Although IMO trading sort of ruined AND enhanced the game.)
If you have some really good skills, you could even make weapons/hats and Valve could possibly add them into the game for everybody!
There are plenty of maps and game modes and there were even special Halloween updates for the last 2 years.
Overall, I would give TF2 a 9.8/10. I just wish people kept trading to trading servers.
Portal
One of my favorite singleplayer games ever.
This game is brilliant. So brilliant that users of the Steam forums had to use old hardware to solve an amazing ARG (Alternate Reality Game).
I've played through this game probably not too many times, but it is definitely one of- if not my absolute favorite.
The boss fight is awesome and pretty challenging for the first 1-2 times.
Overall, it's a bit short but that doesn't stop me from giving it a 10/10. I seriously can't wait for Portal 2 to come out in a few months. (Knowing Valve, though, it will probably get delayed. D: )
So, the Orange Box gets a 10/10 from me.