Hmm... I'd have to say Portal, though, like all of their games, I find it overhyped but still good. Team Fortress 2 and Left 4 Dead were interesting exercises in multiplayer; Half Life a decent FPS but the "innovative storytelling" mostly consisted of refusing to include a developed protagonist. An interesting idea, and not necessarily a bad thing (they did it well) but having a nondescript protagonist is something that countless older games have done with ease.
On the whole, I lump Valve together with Blizzard. They substitute innovation to downgrade the awe-inspiring levels of polish Blizzard has into simply very good polish, but they still make high-quality popular games that defy the release schedules of mere mortals too.