I won't mince words here. I love Valve's original stuff. Next to the Zelda and Mario franchises, I don't think any other games have given me as much enjoyment. Kotaku is a huge fan as well. I love this almost enough to buy it all again at just $4 each. Use code GFDMAY20 get Portal 2 for $4.00 or The Orange Box for $4.00.
Apparently I do love them enough to buy them again— I'm going to get the Orange Box and introduce Half-Life and Portal to a friend.