It has been a long time coming for Sultai to become pretty much the undisputed top deck in Historic, at least from my perspective. I've been working on this deck blend for quite a while, with a high degree of success. The deck is versatile—it's able to constantly evolve and adapt to accommodate for whatever the current metagame may be. Here are some tips for people who may want to play it over the weekend. 

Why I Prefer Sultai to Four Color Control

Let's start with the most common question I get asked every time I bring up the Sultai deck. Four Color Control is absolutely good, and has had a lot of success of late, but I prefer being straight three colors. The first and most important reason should be apparent to most players, and that's the better manabase you get from being Sultai. You have more colored sources for your core colors, and the deck gets to play two copies of Castle Locthwain. 

Castle Locthwain is very important, as a way to get an edge in the lategame, without needing to play an actual spell in that slot. This deck plays a lot of lands, as it is ramp focused, and there are many games where the opponent can answer the first couple threats you play. The deck also isn't terribly threat dense, so having a land that can get you ahead on resources can easily be the difference in a game (versus a deck like White-Blue Control or the mirror). There are also enough ways to gain life that help offset the lifeloss from the Castle. While Four Color Control could in theory play Castle Locthwain, I don't think the mana can really support it.

There's another reason you don't need to play white right now—I don't think Yasharn is at its best in the current metagame. Keep in mind that the reason to play the Four-Color version has almost always just been Yasharn, and little to nothing else. The sacrifice decks are on the decline a bit (after a not-so-great showing over the past few weekends), which is where Yasharn is at its best. In the mirrors Four Color players usually board out their Yasharn's, and try to become a Sultai deck with worse mana and no Castles. I don't recommend this approach. 

While you can argue that both decks are very similar because they play almost all the same spells, I am firmly in the three color camp. The double black sweepers are also really nice, which makes me even more convinced that the three color version is the way to go. (Of course, you're talking to someone who has never been in the Four Color Camp, so take that how you will!)

My Current Recommended List

While I did recently post a sideboard guide for my list from the Zendikar Rising Championship, I want to make some updates moving into the weekend. I will also provide a brief sideboard guide again for some top matchups. Make sure to go back and take a look at my guide from a couple weeks ago if you're interested, and compare the two. Keep in mind metagames are constantly evolving, and guides can become outdated—and sometimes you'll need to make adaptations that don't exactly align with the guide! That said, here's my recommended list:

Creatures of the creature type of your choice get -3/-3 until end of turn.

Vigilance, reach, trample
Whenever Elder Gargaroth attacks or blocks, choose one —
• Create a 3/3 green Beast creature token.
• You gain 3 life.
• Draw a card.

({T}: Add {B}.)

({T}: Add {B} or {G}.)
As Overgrown Tomb enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

Drowned Catacomb enters the battlefield tapped unless you control an Island or a Swamp.
{T}: Add {U} or {B}.

Counter target activated ability, triggered ability, or legendary spell.

Destroy target nonland permanent and all other permanents with the same name as that permanent.

Counter target noncreature spell.

({T}: Add {U}.)

All creatures get -4/-4 until end of turn.

When Uro enters the battlefield, sacrifice it unless it escaped.
Whenever Uro enters the battlefield or attacks, you gain 3 life and draw a card, then you may put a land card from your hand onto the battlefield.
Escape—{G}{G}{U}{U}, Exile five other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Destroy target creature or planeswalker with converted mana cost 3 or less.

All creatures get -2/-2 until end of turn. Exile all creature cards in all graveyards that were put there from the battlefield this turn. If a creature would die this turn, exile it instead.

This spell can't be countered.
Target opponent reveals their hand. Exile all noncreature, nonland cards from that player's hand and graveyard.

{T}, Sacrifice Fabled Passage: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle your library. Then if you control four or more lands, untap that land.

Whenever you cast a noncreature spell, create an X/X blue Shark creature token with flying, where X is that spell's converted mana cost.
Cycling {X}{1}{U} ({X}{1}{U}, Discard this card: Draw a card.)
When you cycle Shark Typhoon, create an X/X blue Shark creature token with flying.

{G}: Exile target card from a graveyard. If it was a creature card, put a +1/+1 counter on Scavenging Ooze and you gain 1 life.

Each opponent can't draw more than one card each turn.
−2: Look at the top four cards of your library. You may reveal a noncreature, nonland card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Destroy target creature if it has converted mana cost 2 or less.
Revolt — Destroy that creature if it has converted mana cost 4 or less instead if a permanent you controlled left the battlefield this turn.

Target player reveals their hand. You choose a nonland card from it. That player discards that card. You lose 2 life.

Castle Locthwain enters the battlefield tapped unless you control a Swamp.
{T}: Add {B}.
{1}{B}{B}, {T}: Draw a card, then you lose life equal to the number of cards in your hand.

Whenever you tap a Forest for mana, add an additional {G}.
+1: Put three +1/+1 counters on up to one target noncreature land you control. Untap it. It becomes a 0/0 Elemental creature with vigilance and haste that's still a land.
−8: You get an emblem with "Lands you control have indestructible." Search your library for any number of Forest cards, put them onto the battlefield tapped, then shuffle your library.

({T}: Add {G} or {U}.)
As Breeding Pool enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

When you cast this spell, you gain half X life and draw half X cards. Round down each time.
Flying, trample
Hydroid Krasis enters the battlefield with X +1/+1 counters on it.

({T}: Add {U} or {B}.)
Fetid Pools enters the battlefield tapped.
Cycling {2} ({2}, Discard this card: Draw a card.)

({T}: Add {B}, {G}, or {U}.)
Zagoth Triome enters the battlefield tapped.
Cycling {3} ({3}, Discard this card: Draw a card.)

Creature cards in graveyards and libraries can't enter the battlefield.
Players can't cast spells from graveyards or libraries.

Draw a card. You may put a land card from your hand onto the battlefield.

Choose odd or even. Exile each creature with converted mana cost of the chosen value. (Zero is even.)

({T}: Add {G}.)

Counter target creature spell.

Counter target spell with converted mana cost 4 or greater.

This list is highly adapted toward a Control and Midrange based format. If you're expecting more sacrifice and aggro decks, we need to add a removal spells or two back into the maindeck. Each time you add in a card like Fatal Push or Extinction Event to the maindeck, it will in turn make the matchups against the mirror and White-Blue Control worse. It's difficult to strike a balance, and this is what I have currently settled on. Every single matchup is winnable—in fact, I'm not sure there is a bad matchup that actually exists for this deck!

The biggest addition is Tale's End, which is great against the mirror, Goblins, White-Blue Control, and more. It has utility in many matchups, and really shines in others. The deck wants to have a critical number of counterspells. I have been trying my best to improve the Goblins matchup, as with my previous list, that was the matchup giving me the most problems. In order to make room for Tale's End, I ended up removing Aether Gust. 

This will come as a surprise to a lot of people. I went from advocating playing four Aether Gust to not playing any at all! One of the biggest issues is the Goblins players relying heavily on Herald's Horn. Aether Gust isn't great against this card, as putting a spell on top of their deck can be much worse than countering it. Also, because you have Tale's End, that means more answers to Nissa and Uro, so with those bases covered, Aether Gust no longer holds much appeal. Finally, it's a dead card in certain matchups, which doesn't help its case for inclusion. I still wouldn't play more than two Tale's End, as there are some matchups where it counters little, or nothing at all.

The other main place where I made changes is the sideboard. I'm definitely considering the Thought Distortion slot, although I do like keeping it for a discard spell. (For me it's either Distortion or Duress.) I also think that with the prevalence of Goblins, you need some good sweepers against that strategy. Witch's Vengeance is the best one, but Languish isn't that much worse, and can be very strong in other matchups too. I've gone with the split between those two cards. 

I like Grafdigger's Cage and Scavenging Ooze as hedges towards some of the less popular decks and Sacrifice strategies that can show up. That doesn't mean I board in Scavenging Ooze in every matchup, though it has utility in a lot of spots. I'm only doing the top few matchups here, but I hope this helps players preparing for a Qualifier this weekend, or looking to pick Sultai up for the first time. 

Mirror/Four Color

IN:
2 Negate
1 Thought Distortion
3 Narset, Parter of Veils
2 Shark Typhoon

OUT:
2 Fatal Push
1 Extinction Event
1 Cry of the Carnarium
1 Disdainful Stroke
2 Maelstrom Pulse
1 Uro, Titan of Nature's Wrath

UW Control: 

IN:
2 Negate
1 Thought Distortion
3 Narset, Parter of Veils
2 Shark Typhoon

OUT:
2 Fatal Push
1 Extinction Event
1 Cry of the Carnarium
1 Disdainful Stroke
2 Maelstrom Pulse
1 Uro, Titan of Nature's Wrath

Control and Sultai are the same gameplan, the games are usually grindy, and Shark Typhoon is critical, as it's important to keep up mana on the opponent's turn, for either a counterspell or to represent a cycled Shark Typhoon. The removal spell you lean on here is Eliminate, since it answers Narset and a Shark Typhoon. 

If you feel the need to keep in a Maelstrom Pulse as an answer to Tefer, I think that's okay too, though it can be difficult to find cuts. The one card that messes up the plan of not doing much on your own turn is Thought Distortion—that card can break a game on its own.

Goblins:

IN:
1 Cry of the Carnarium
1 Essence Scatter
1 Witch's Vengeance
1 Languish
1 Elder Gargaroth

OUT:
3 Hydroid Krasis
2 Shark Typhoon

As tempting as it is to cut Maelstrom Pulse or Thoughtseize, resist the  urge! Answering a turn three Herald's Horn is your first order of business, followed by having enough counters to stop the Muxus. After sideboard, you rarely lose to the little creature beatdown plan of Goblins if they can't get ahead on cards. 

Sacrifice

IN:
1 Grafdigger's Cage
1 Scavenging Ooze
1 Cry of the Carnarium
1 Elder Gargaroth
2 Negate
1 Languish

OUT:
2 Tale's End 
1 Disdainful Stroke
2 Shark Typhoon
2 Thoughtseize

Even though sacrifice decks are on the decline, they regularly come and go. Given time, I expect they'll pick up again. 

Paradox Engine Combo

IN:
1 Grafdigger's Cage
1 Scavenging Ooze
2 Negate
3 Narset, Parter of Veils
1 Cry of the Carnarium

OUT:
3 Hydroid Krasis
1 Shark Typhoon
1 Extinction Event 
1 Disdainful Stroke
1 Forest
1 Uro, Titan of Nature's Wrath

This deck is the new kid on the block, but it's a pretty favorable matchup for Sultai. There is no single card to worry about. The easiest way to lose is the opponent getting to untap with Emry or a planeswalker in play.

Should Uro be Banned?

This question seems to be gaining some more steam just over the past week or so. I was shocked that so many people registered Sacrifice decks for the Zendikar Rising Championship, as I have long thought Sultai was the top deck in Historic. In my opinion...yes, we will need an Uro ban at some point. Maybe it's now, or in a month or two. I suspect the longer the format exists without an Uro ban, the more the ban chatter will increase. If we are at the point where Sultai is the undisputed top deck in the format, then it's not hard to ban Uro. We know how insanely overpowered the card is. 

I suspect that before a decision is made, WOTC will take data into account from this weekend's round of qualifiers on MTG Arena, as they should! Sultai is by far the most popular deck on the ladder these days, and we're approaching a spot where the format is mostly solved. 

However, Sultai isn't that much better than the other decks. If it does get banned, it could just cause all the other decks to move up one slot in the metagame, but not actually allow an opening for many new decks to shine. We'll see soon enough. It could be that combo decks would be better positioned if the ban does happen.