I recently DL the 'orange box' from steam and love it. Portal is quirky and fun. Half Life 2 has excellent story-telling and an atmosphere unlike any I've experienced since System Shock 2. Team Fortress 2 is just, well, wacky, violent, and encourages team work . Make of that what you will.
I'll second that vote for Portal. Talk about atmosphere, I still carry around the end theme song (Still Alive) on my iPod lol because it's so hilarious and brings back such fond memories.
__________________ Men do not stop playing because they grow old. They grow old because they stop playing. -- Oliver Wendell Holmes