If I only get to choose one, then I guess the Orange Box, primarily because I was a huge fan of the Half life series and they're one of the foundations of gaming today, and with the popularity of Counter Strike, it's a given.
Spore is a frickin awesome idea, and really the only truly great game up there. I love being able to develop your character however you want it to be developed, but there is always a slight doubt as to what will happen in it's evolution. That is probably one of the best PC games out there. If only it were less expensive...