The Portal games are some of the best games in the world. I won't say they're my favourite games of all time, but the design work is absolutely incredible, and they meet game "goals" extremely well; both have clear cut objectives, there's challenge and failure, the player knows what their options are, there's skills to improve and concepts to learn and the immediate feedback is just fantastic.
The Half-Life franchise is in close second, but I just feel that the FPS gameplay pales towards the puzzle nature (especially cooperatively) of Portal. The Counter-Strike series I probably have the most time in (not logged), but it has quite the difficulty curve.
The other Valve games aren't bad; they're just not my cup of tea.
Poll