Portal....

Portal had to be the best FPS (without the shooting) that I've ever played. The wit was outstanding... and honestly, There IS cake! The later levels take not only a good mind but timing. All in all, I HIGHLY recommend getting this game whether it's free through Steam (til May 24th, 2010) or you get a bundle package that contains it. Get it, don't install any other game until you have beat it.

Just remember, You will be baked, and there will be cake.