I voted Tetris, but you guys make really good arguments for Portal.
Of course, genres are a weird thing. Why isn't every game where you play as a character an RPG, even though you are playing a role? Why is Tetris accepted to be a puzzle game even though you can't actually "solve" the "puzzle"? Who actually plays party games at parties? If I were to throw a Hail To The Chimp party, would anybody come? ... Please? I'm lonely. Hail To The Chimp is all I know. Guys? What about Fusion Frenzy?
... What were we talking about?