You can't talk about something being the 'best' without breaking it down in to separate categories like graphics, sound, value for money, expansion possibilities, storyline, replay value, etc. This is a silly poll that serves no purpose.
You could say TF2 is Valve's best work because it might bring in the most revenue. Or you could say Portal 2 is their best work because it's the most polished Source game yet. You could say Dota 2 is Valve's best work because of the massive, multi-million $ competitions. Why not even say Half-Life 3 is their best work because it's taking so long to release?