Which game's storyline is best?

Portal and Portal 2
Beyond: TS too.
And Heavy Rain!

PONG, sooo good, I hope they make a movie out of it.

Beyond: Two Souls, The Walking Dead, The Last of Us and more ....

Bastion, Heavy Rain and Beyond: Two souls!
They have some of best stories I have personally seen in a long time, they somehow just drew me in and wouldn't let me stop playing xD
I just needed to see how everything would turn up haha :P

Street Cleaning Simulator by a long shot. Rich storyline and tons of emotional depth.

Sid Meier's Alpha Centauri

"Amnesia: The Dark Descent"

Usually I loose interest in the story of games fast and just start playing "optimally", but that game really kept me playing the story for a long time.

Metal Gear Solid 3 and Assassin's Creed 2

Even though it's a bit short, I vote for Brothers: A Tale Of Two Sons. Even though Beyond: Two Souls is awesome too!

Have to say Portal 1.

No other human being, just a voice from the off, mostly sterile rooms, barely anything is told, yet the story is very intense. I think I never immersed so deeply into a game before or after.

Mafia II

Even though most wont agree I loved Chaos Legions ending, New Vegas is great also, Arc The Lad 2 was also great.

Plenty of good stories in games, to many to list.

The Witcher 2.

To The Moon.

Red Dead Redemption, although there are many other good stories.

Metal Gear Solid 3, Kingdom Hearts 2, The Last of Us, Too many!

Gothic 1, Gothic 2, Gothic 3 (3 is not that good tho)

The Line, Torment, Fallout, Thief, Grim Fandango, TLJ, Mask of the Betrayer...

Alan Wake

System shocks, Deus Exs, Gabriel Knights, The longest journeys, Omikron: The nomad soul, Fahrenheit, Broken Swords, Mass Effects, Knights of the olds republics, Sleeping dogs, I have no mouth and i must scream, Max Paynes, Sam & Maxs, Kyrandias, Vampire: Bloodlines, Legacy of Kains,

and others i guess :)

I loved "Final Fantasy VIII", "The Cat Lady", the "Half-Life" saga, the "Fallout" saga, and pretty much any good rpg-ish story :)

Sacrifice and Shadowman etc :) ( by Shiny Entertaiment and Akklaim/Acclaim Entertaiment) get them on Steam ;)

The first Bioshock. Posibly Alan Wake and Portal as well, although they do not have fantastic stories, but rather great ways of telling them.
Other than those, I cannot remember another game I've played with a remarkably great story.