Pathfinder: Kingmaker Walkthrough and Guide by chautemoc[www.neoseeker.com] - A complete game guide. Besides a walkthrough there is pretty much everything you could want from a guide: builds for companions, builds for you, a guide for unfair difficulty, achievements guide, party management, DLC guides, and much more. I will link subcategories of this huge guide to help in navigation in future sections, but if you want to navigate directly from the source this is it.

Guide and Walkthrough by kimagure[gamefaqs.gamespot.com] - This is the closest thing there is to a 100% walkthrough of the game, it's PERFECT and the writer did some serious research. It tells you every little thing in every area, every loot piece, every choice, every stat of every enemy, their saves, AC, everything. When I say it's perfect I really mean it. Sadly the guide is still in the works and only covers up to the end of Act 3, but it's pretty much the best around until that point. It also offers a lot of information on the side like tutorials on base game features.
EDIT: now includes also Act 4 and 5![gamefaqs.gamespot.com]

Guide and Walkthrough by chris-williams[gamefaqs.gamespot.com] - This guide is also really good. It is more akin to a full manual of the game, together with a guide and walkthrough. It explains very well each class, each companion, it gives you some builds, also tutorials and some tips, then throws you in the game with a very detailed walkthrough, tough not as much as kimagure's. It is a bit hard to follow due to how it is structured, but this one has the whole game in it. It also has a list with every item in the game. I don't need to tell you that this thing is HUGE.

RangerTony's Tutorials playlist - Tutorials basic and not so basic explained with great effort and care, in depth and not in an overwhelming way. New players, especially not familiar with Pathfinder should watch all of these, they are not that long. More experienced players should watch them too because there is always some knowledge to be gained from the guy, who seems to know his DnD stuff pretty well.

Gamerguides' walkthrough[www.gamerguides.com] - It is an acceptable walkthrough, with pretty much all you might want and some builds. It lacks strong community interaction and thus it stays radicalized in the ideas of a small group of people, instead of the community effort to build something that is a product of many minds, which many other guides have.

InEffect's builds compendium[www.gog.com] - A 60+ pages worth of meta knowledge, the first page also has a lot of builds by the guy, for main char, mercenaries and companions. It also has some considerations on various aspects of the game.
InEffect's builds are a min/maxer power gamer dream, and most of them do well even on Unfair, not all are conceptually appropriate but they sure are effective.

Roahin's companion builds[www.gog.com] - Various companion builds by Roahin.
Roahin's main char builds[www.gog.com] - Various main char builds by Roahin.
Generally Roahin's builds are more suited to roleplayers, they are not min/maxed but they hold their own on the harder difficulties, though some of them may struggle on Unfair.

Neoseeker chautemoc's builds[www.neoseeker.com] - These are all builds from various authors, for pretty much everything you want, be it main char, companions or mercenaries. Most of these are made by InEffect, but there is some diversification and it's not always clear who made what.
In this index of builds chautemoc manages to find a middle ground and put many builds that more or less make happy everyone, because of the big variety. This comes at a price, though, at times builds may not be clear, be outdated or contain mistakes and since they didn't write them it's difficult for them to modify the builds accordingly.

Rockspecth's builds compendium[pastebin.com] - 1 build for Harrim, 1 build for Jaethal, 2 builds for Amiri, 1 main char build.
Rockspecth's builds are middle ground on min/maxing, there is not a whole lot of them but those few are pretty good and can hold up even on Unfair, while also being roleplay-wise accurate. Generally they improve the Neoseeker builds.

Single-Class Build Series by JPDG - Some builds that are single class only (no dipping or multiclass), and most of these are companion respec that require a mod, but can be done on main char too. There is also links to all his other guides, which address other matters like a 30 days act 1 guide.
The builds try to min/max what they can from a roleplayed single class, it's a fun challenge and some of them are very good, but harder difficulties will be tough with these if used on main character.

Brother Mutant73's builds playlist - An impressive 100+ videos playlist each containing an original, fun and powerful build for you or your companions. The content may not be the most easily accessible due to the videos spanning from 30 minutes to a couple of hours EACH, but there is good info in them and the guy is nice to listen to.
Those builds are not min/maxed, they are tailored to roleplay mostly, they abstain from too much dipping in other classes and place fun and experimentation above power gain. That said, they still remain powerful and could even serve you well in Hard/Unfair if you know how to play.

Kingdom Management by chris-williams[gamefaqs.gamespot.com] - It has a spreadsheet look and it's easy to understand, it contains really everything down to the most minuscule detail of number crunching, all events, all advisors events, artisans etc.

Kingdom Management Guide by kimagure[gamefaqs.gamespot.com] - It details all you might want to know about kingdom management, advisor events, artisan quests and more, but it lacks some things the first one has, i.e. projects, building informations, cities numerical details.

Kingmaker Complete Artisan Overview (by Ejnar Håkonsen)[docs.google.com] - A easy to navigate and understand spreadsheet about artisans.

Unfair Guide by InEffect[www.neoseeker.com] - Pretty much your Unfair encyclopedia by the best Unfair player. Party management, builds, somewhat of a walkthrough up to Act 2 end, viability of classes and cheese strats.

Tryharding through Kingmaker, a guidebook to hell, by Pink Eye.[rpgcodex.net] - It teaches you in detail and with pics how to approach Unfair encounters up to the start of Act 2, even with a suboptimal party.

Victor Creed's Unfair solo playlist - Various complete playthroughs, with and without bugs/cheese/cheats, on Unfair, SOLO. You can learn a lot by watching these videos, but because of the length you may want to skip to the parts you are most stuck in.

MONO RPG's Unfair playlist - Another Unfair playthrough, albeit done with some cheesy tactics (5 sylvan sorc mercs really?), still a good reference if you want some ideas.

Romance (approval) save file shadoozling tutorial by ThatGuy642 - In this reddit post is explained in detail how to access the save file and check/modify the approval of companions without needing mods. It's a very handy resource if you lose track of it.

• I will update the guide if I find something worth noting, suggestions are welcome, even your videos or written stuff if you have something worth showing.
  
  
• If you are one of the authors of what's linked here and do not want your work featured, please let me know.
  
  
• I realize after going on for a bit that this is starting to look more like me plugging content creators instead of helping people, in my defense know that I only plug good content creators.
  
  
• This started as a personal project to just have all the stuff in one place, but then I figured since I've done it might as well make it public, hence the takes on each of the links are my personal opinion and should not be taken as objective statements.