Um... scratch that Orange Box entry. I am currently on a gaming high as I've just had one of the best gaming experiences of my life. My life, I tell you!
This game is a 3-hour experience like no other. And the end-game is nothing short of gaming genius. I hate to spoil the finale for you, but the game actually sings to you at the end. I was so shocked and impressed by this that... well... I just can't stand it... it was so awesome.

Portal starts out as an enigma. You are like, WTF is this? What is going on. Then comes the "Ah ha!" moment and you begin to get into the game. Then it becomes a real mind-bender. Then you suddenly feel invincible as you master the concept of the game. Then suddenly you are fighting for your very life. Then you have a song sung in your honor. So freakin' awesome.
Portal is a puzzle game that looks like a 1st person shooter. And that's a good thing! It's a pure puzzle game through and through. It's a puzzle that, for the most part, you've never played before. It is extremely tactile and satisfying.

I just can't say enough about it. It is a game that will forever live in my memory as a personal key moment in gaming.
p.s. I <3 my weighted companion cube
p.p.s. And did I mention that Steam is, like, the best feakin' thing ever? Holy crap Valve did such an incredible job bringing it together. Everything about it just feels great and games are so accessible. If you don't know what I'm talking about, you just have to ask me for an explanation or go install a recent Valve game.
heeellllooooo. Are you still there?
http://www.portalprelude.com/