The Witcher 3: Wild Hunt gives players the choice of either going in with a clean slate, or answering a few questions about the part they played in the events of the previous game.

1. Choices From Witcher 1 2 3
2. Witcher 3 Choices From 2 11
3. The Witcher 3 Choices From Witcher 2

The Witcher 2: All Major Choices - A Definitive Guide (+ the "Best" Ending & Geralt Archetypes)
Given that there's constant confusion over the '16' endings and what choices have significance, I 'have compiled a list,' as our favorite Nilfgaardian ambassador would say. Every choice listed ignores the minor stuff - killing trolls, Adam Pangratt, Malena, etc. - and focuses on choices with important political and cultural ramifications. At the end follows a heavily-discussed conclusion on what ending, all morality aside, leaves the most stable North to face the Nilfgaardian invasion.
Prologue
SPARE ARYAN: He survives through to Chapter 3 and La Vallete Castle remains Temerian.
KILL ARYAN: La Vallete Castle becomes Nilfgaardian, surrendered by Mary Louisa La Vallete.
Chapter 1
GIVE SWORD: Hand Iorveth his sword and a nonhuman pogrom occurs in Flotsam. Margot lives. Derae dies. You can get to the barge on Iorveth's path with a prisoner farce or a frontal assault.

Choices From Witcher 1 2 3

HELP ROCHE: Iorveth is captured and a festival occurs in Flotsam. Margot dies. Derae lives. You can only get to the barge on Iorveth's path with a frontal assault. Although this is a literal slap in the face to Iorveth, ironically helping Roche guarantees the survival of all the civilians of Flotsam besides Margot.
CHOOSE IORVETH: Abandons the Temerian cause for the Pontar Valley. Dethmold is now guaranteed to die, and Temeria will always be divided, and Henselt's invasion will automatically fail. Unlocks SAVE MOTTLE/PURSUE LOREDO
CHOOSE ROCHE: Abandon nonhumans for the Temerian cause. Vergen is guaranteed to fall, and Stennis and Loredo are now guaranteed to die.
SAVE MOTTLE: Save the elves from the tower to get sexytime with Mottle later on, and earn Iorveth's gratitude. Loredo sells Flotsam to Kaedwen.
PURSUE LOREDO: Chase Loredo into the town square and butcher him. Mottle and her friends die and Iorveth is bitter. Loredo perishes and Flotsam stays Temerian. Pursuing Loredo does kill two birds with one stone - accomplishing the objective of Roche's path while allying yourself with Iorveth.
Chapter 2
Iorveth Path
SPARE STENNIS: Stennis never faces justice and becomes King Stennis I of Aedirn, stabilizing the country and opposing Saskia's Free Pontar.
LYNCH STENNIS: Stennis dies, Aedirn collapses into anarchy, but Saskia's new nation is unopposed.
Roche Path
KILL HENSELT: The Blue Stripes get vengeance, but Kaedwen collapses into chaos. The eastern half of the North is an anarchic mess.
SPARE HENSELT: Roche is bitter, but Geralt gives him a bro pep talk and Kaedwen stays a strong Northern realm under the cured Henselt. Actually the strongest if NATALIS is chosen later on.
Chapter 3
SAVE TRISS (Iorveth): The Nilfgaardian plot is destroyed, with the death of Renuald, Shilard and their entire party. The Council and Conclave is successfully restored, assuring a strong Northern network of magic users. Temeria is partitioned or dissolved. Iorveth is horribly wounded.
GO WITH PHILIPPA: The Nilfgaardians start a witch hunt and all mages are implicated. However, Saskia establishes her authority as an unopposed Queen of the Free Pontar. Iorveth is Geralt's best bud, and Triss survives anyways.
SAVE TRISS (Roche): The Nilfgaardian plot is destroyed, with the death of Renuald, Shilard and their entire party. The Council and Conclave is successfully restored, assuring a strong Northern network of magic users. Temeria is partitioned/dissolved. Dethmold escapes, Roche takes Anais into exile. This is the only gamestate where Dethmold survives.
SAVE ANAIS: The Nilfgaardians start a witch hunt and all mages are implicated. Roche and Geralt are bros. Triss survives anyways. Dethmold dies horribly. Temeria is guaranteed to avoid dissolution. Unlocks GIVE ANAIS TO RADOVID/GIVE ANAIS TO NATALIS
GIVE ANAIS TO RADOVID: Radovid takes Temeria, whether Henselt is dead (no resistance) or alive (Henselt is extremely flummoxed). He claims he is merely holding it as a protectorate, but it essentially becomes part of his dominion, and Anais will likely never be Queen. If Adda is dead, Radovid betroths himself to Anais and closes his grip on Redania and Temeria firmly. Roche accepts this turn of affairs, and takes up the Redanian Red posthaste.
GIVE ANAIS TO NATALIS: Temeria survives independently, Anais aspires to be a Warrior Queen, and all Temeria, from Vizima to La Vallete Castle bows down to her. Roche remains a Blue Stripe.
SAVE SILE: Sile tells you Yennefer's location and departs. The Lodge retains a strong member.
LET SILE DIE: Sile dies horribly and the Lodge is terribly crippled.
SPARE SASKIA (Iorveth-Save Triss): The Lodge is strengthened, able to retain control over Saskia, with the fleeting hope of using the dagger Iorveth acquired at some future point. The realm of the Pontar Valley retains its leader, despite being influenced by the Lodge.
KILL SASKIA (Iorveth-Save Triss): The Lodge's plans are utterly wrecked, their trump card destroyed forever. However, the last dragon in the world is likely dead, and the Pontar Valley lacks a leader.
DISENCHANT SASKIA (Iorveth-Go With Philippa): Saskia is freed of the spell, the Lodge's plans ruined and their captive unshackled. The Pontar Valley stays strong and is truly free of outside control. Geralt earns the gratitude of Saskia and Iorveth alike.
SPARE DRAGON (Roche-All): The Lodge retains its weapon, whose sole purpose is to serve their bidding as a weapon of destruction given the fall of Vergen.
KILL DRAGON (Roche-All): The Lodge loses its weapon, and Kaedwen need not fear a counterattack by a vengeful Saskia on its newly conquered Lormark.
SPARE LETHO: Foltest gets no justice and Geralt breaks his promise to Roche. The School of the Viper may spring up as a powerful tool against the Wild Hunt, or Nilfgaard may try to use Letho more in the future. Letho and Geralt settle their debt of friendship and blood.
KILL LETHO: Geralt disregards Letho's perspective and delivers bloody justice for Foltest and keeps his oath to Roche. The School of the Viper is likely doomed and Nilfgaard loses a potential pawn.
One path for greatest Northern stability is as follows (with a couple *minor choices tossed in due to their relevance). Credit to Lord_Roose for promoting this path for Geralt. Note: you may have conflicting opinions about the path of most stability for the North, and obviously this will be reworked once Witcher 3 is released and people have seen how the whole thing plays out. For the time being, I'll leave this up here - obviously after TW3 there will be choices that didn't matter or choices we couldn't anticipate mattering - but until that time, here is one path that arguably establishes the strongest overall North to weather to coming storm.
SPARE Aryan so La Vallete stays a Northern property.
Help ROCHE so Flotsam stays a stable, functioning town.
Side with IORVETH (Dethmold inevitably perishes, eliminating a rogue necromancer a danger to all around him.)
SAVE Mottle so Kaedwen's dominion increases to include Flotsam.
SPARE Stennis so Aedirn becomes a strong nation again.
*KILL the Nilfgaardian mages at the end of Secrets of Loc Muinne so Nilfgaard loses all 4 of its mages in the North over the course of the game: Vanhemar, Assire var Anihid, Adalbert and Cynthia.
SAVE Triss so mages spread throughout the North in positions of power and support, boosting every realm's government.
KILL Sile so the Lodge is crippled and causes less future chaos.
KILL Saskia so the Lodge loses its trump card and Vergen falls back into the hands of the rightful King Stennis.
*LEAVE Iorveth to die in a Kaedweni jail so the Scoia'tael are less effective at undermining the Northern Kingdoms.
KILL Letho so that Nilfgaard cannot use him in further endeavors, and there is nobody to resurrect the School of the Viper - for we've seen how Nilfgaard uses witchers against the North.
IN SUMMARY, we have

• a strong Redania under Radovid gaining new Temerian territory,
• a strong Kaedwen under Henselt gaining Flotsam and Temerian territory,
• a La Vallete bastion against Nilfgaardian incursion,
• a crippled Lodge and strong, loyal network of mages supporting the monarchies of the North,
• a united Aedirn under the new King Stennis I to form a bulwark in the southeast,
• a suppression of nonhuman dissent, and
• the ENTIRE Northern Nilfgaardian expeditionary force slaughtered to a man - Renuald, Shilard, Assire var Anihid, Cynthia, Adalbert, Vanhemar and all their retinue.

***not saying that this is the most SATISFYING or HAPPY ending for the characters - it stays pretty darn neutral and screws over a lot of people. I myself like siding loyally with Saskia, or restoring Anais to the Temerian throne, but those paths leave a weaker North overall against the onslaught of the Black Ones.
If you're looking to do another playthrough and want to characterize your Geralt in a fresh new way, try out these character archetypes as a basis.
GERALT, FREEDOM FIGHTER: This Geralt fights for nonhumans and freedom. He is benevolent and patient.
Prologue
SPARE ARYAN: You sympathize with his cause.
GIVE SWORD: Iorveth's your bro.
CHOOSE IORVETH: Roche's vendetta is petty.
SAVE MOTTLE: For the people.
Chapter 2
LYNCH STENNIS: Aedirn is free.
Chapter 3
GO WITH PHILIPPA: Only way to save Saskia, Free Peoples' Queen.
LET SILE DIE: Sile and the Lodge seek only to control Saskia and pervert the freedom of the people.
DISENCHANT SASKIA: Duh.
SPARE LETHO: His murder of kings ultimately strengthens the freedom of Saskia's new realm and anyone like her. No reason to kill.
GERALT, WAYWARD WITCHER: This Geralt has no time for any distractions save for his Witcher duties and romantic interests. He does not maintain allegiances and will readily kill his opponents.
Prologue
KILL ARYAN: He challenges you and you show him his foolishness.
Chapter 1
HELP ROCHE: Iorveth has given you little reason to trust him.
CHOOSE IORVETH: Seems like the fastest way to save Triss and find the Kingslayer.
PURSUE LOREDO: Can't leave any loose ends.
Chapter 2
SPARE STENNIS: A Witcher has no place inciting riots.
Chapter 3
SAVE TRISS (Iorveth): Triss is your goal, your rock, and your steadfast ally. She cannot be abandoned.
SAVE SILE: You need to resolve your quest for Yennefer. Sile's tidbit of info is worth letting her live.
KILL SASKIA (Iorveth-Save Triss): Witchers do not kill dragons, but she left you little choice.
KILL LETHO: Witchers shouldn't play politics, and if anyone's going to do it, it's going to be Geralt alone.
GERALT, TRUE TEMERIAN: This Geralt is the savior of Vizima and friend to Foltest. He is honorable and interest in the survival of the Temerian state above all.

Witcher 3 Choices From 2 11

Prologue
SPARE ARYAN: Temeria must stay strong and united.
Chapter 1
HELP ROCHE: Iorveth is a danger to the state.
CHOOSE ROCHE: Roche is Temeria's last hope.
Chapter 2
SPARE HENSELT: Roche must not dishonor Temeria with more kingsblood.
Chapter 3
SAVE ANAIS: Last hope for Temeria's bloodline.
GIVE ANAIS TO NATALIS: Temeria must not splinter.

The Witcher 3 Choices From Witcher 2

SAVE SILE: Dethmold was your enemy, not Sile.
SPARE DRAGON (Roche-All): The Dragon helped Aryan once, it might help him again. Aryan is wild but still a Temerian.
SPARE LETHO: Killing him will not bring Foltest back. And if Anais wishes to avenge her father one day, Letho will still be out there.
GERALT, BUTCHER OF BLAVIKEN: This Geralt is not human but a Witcher. He is bred to be a killer, and revels in combat and violence.
Prologue
KILL ARYAN: Nobody challenges a Witcher.
Chapter 1
HELP ROCHE: Nobody commands a Witcher.
CHOOSE ROCHE: Loredo must die screaming.
Chapter 2
KILL HENSELT: Blood for blood.
Chapter 3
SAVE TRISS (Roche): The Nilfgaardians must pay.
LET SILE DIE: Nobody orders the death of a Witcher.
KILL DRAGON (Roche-All): Nothing can stop a Witcher.
KILL LETHO: All crimes end in blood.