Which decision......

[Dryhtscipe]

If we're not limiting to the timescale of the books themselves, then blame filthy Aegon I!

[SerJeremiahLouistark]

"Dany Jon or Bust" I understand your logic but couldn't the incest between Jaime and Cersei be credited with the saving of Kings Landing? If Jaime and Cersei aren't bumping uglies, Jaime doesn't become Kingsguard to be closer to her, in turn isn't there to kill Aerys before he wildfires the whole city. But then again, if Aerys wildfires the city, maybe Robert rules from Dragonstone and all the Lannisters die.

[Lord_Trooper]

John Arryns decision to bring Littlefinger to KL and /or investigate the Cersie / Jamie relationship.

[Arya kiddin']

Just go back all the way people. Aegon's conquest was the one decision.

[Jon's Queen Consort]

Rhaegar's decision to disappear with a teenager.

[TalalOfDorne]

I dont know which had the MOST impact. But I would like to point out that Cersei "boaring" Robert could be one. Roberts dead, so Stannis, Renly and Joffrey try to take the crown. And with Robert out of the picture, the Starks had zero safety in KL. Meaning the Lannisters had the liberty to take any action they wanted against the Starks and Tullys. Whether Ned was executed or not, Robb still marched south.

Also, one could argue that Cersei wouldn't have done what she did had Ned not told her what he knew. So it could be on Ned and Cersei.

[VarysBaelishTyrion]

Danerys deciding to walk into a fire with her dragon eggs, now has the power to potentially conquer and rule Westeros, Essos and destroy the white walkers.

[The Answer]

"Dany Jon or Bust" I understand your logic but couldn't the incest between Jaime and Cersei be credited with the saving of Kings Landing? If Jaime and Cersei aren't bumping uglies, Jaime doesn't become Kingsguard to be closer to her, in turn isn't there to kill Aerys before he wildfires the whole city. But then again, if Aerys wildfires the city, maybe Robert rules from Dragonstone and all the Lannisters die.

In reply: It seems Jaime is madly in love. Lol, I think that is pretty clear. "The things I do for love....".... Wooooosh, is a perfect example of that. Jaime did join the KG to be closer to Cersei, but I think it's clear that their love for one another began at a young age, and Jaime wouldn't let anything get in the way of them being together, not even Casterly Rock.

This is definitely interesting though. I mean sure, Aegon the Conquerors decision to... wait for it.. 'conqueor' Westeros is a given. But if we look at the time since The Tourney at Harrenhal, things get interesting.

I initially said Jaime & Cersei's relationship.

Others said the manipulation of Lysa, leading to Arryn's death.

Also presented, Prince Rhaegar's selection of Lyanna Stark.

If Rhaegar selects Elia Martell, Jaime and Cersei are still in love, and LF still desires The Vale of Arryn. Additionally, most likely, Rhaegar still would seek Lyanna out and eventually be discovered.

If LF never plots Arryns death, then Arryn still would have been intrigued by the Jaime/Cersei relationship and would have most likely died anyway. Ned Stark would then be named HoTK, and would also be intrigued by the J/C relationship. This leads to his death, and the plethora of death thereafter.

If Jaime and Cersei are born without an ounce of attraction towards one another, Arryn never becomes suspicious of the matter. However, LF would still plan Arryn's death. Ned would be named HoTK, but now with a much better chance of survival. Ned lives, as does Winterfell, and the young wolf never becomes "King in The North (at least, not for reasons of vengeance)."

I'm sticking with my gut feeling for now. Someone persuade me otherwise!

[Lady Nastja]

I would have to say Lysa's decision, following prompting by LF, to kill Jon Arryn.

[Swiftraven]

Probably not even close to the biggest per se but just throwing it in there:

Ned's decision to let the Stark kids keep the direwolf pups.