The first thing I played was the original Portal that I had collecting dust on my shelf. I can't believe I never played it before, it was very good & had some offbeat Paranoia-style humour that made it a fun game, especially given it's age.
I even went through some of the developer's commentary and when I hit the end credits, I loved the tune so much I tracked down the Youtube video of them. Fantastic!
So now I've just finished installing Portal 2 and I'm about to dive into the single-player levels in the next few minutes.