Best game "ever" implies that it's the best game that has ever existed, exists now, or will exist in the future for every gamer in the world at all times in all ways etc. While it was interesting enough to get me started, fun enough to make me keep playing it, challenging enough to make it a... challenge, and atmospheric enough to make it more than just a puzzle/shooter, I can't in good conscience say that it's the "best game ever' because it's not. There's at least someone, somewhere, at some point, who thought that portal was (at best) meh. I disagree with them, but their opinion matters just about as much as mine, so it should be taken into account too.
Portal is a fantastic game that seamlessly marries the mechanics of a shooter from the first person perspective (someone should really come up with a name for that) with the brain teasing of a puzzle game. Combining that with Valve's superior writing department and level design, and you have some sort of a meta-game that, while not the best game ever (I refer to my previous paragraph), is so fantastic that everyone should try it at least once just to see for themselves if they like it.
...... And I pray to every god I can find on Wikipedia that Portal 2 doesn't kill the franchise.