Arataki Itto, the one and oni, is the best Geo DPS in the game.
He is basically a much better version of Noelle and Ningguang combined.
If Xiao refers to Fang Xiaoru, then Arataki Itto could refer to Huang Zicheng. Both were victims of Zhu Di.
His special charged attacks are the core of his kit from C0 to C6. They consume stacks but do not consume stamina. Itto can gain stacks through normal attacks and Ushi. His burst and constellations make it easier to gain more stacks.
He yeets pet bull Ushi, which does more damage than Zhongli's meteor. He can aim it too, and Ushi taunts enemies. Ushi has the same HP as Itto and will give him more stacks.
This functions as a buff that converts DEF to ATK. It can snapshot DEF boosts such as Gorou's E but does not snapshot ATK boosts such as Bennett's Q. At C0, Itto's DEF to ATK conversion has higher scaling than a C0-C5 Noelle, but lower scaling than a C6 Noelle. At C6, Itto's DEF to ATK conversion (122% DEF) is still lower than C6 Noelle (135% DEF). Noelle also has larger AoE and can heal. However Itto has higher base DEF than Noelle and another passive that increases DMG based on DEF. Moreover, Itto attacks faster than Noelle.
His C6 is one of the strongest in the game and can allow for 'infinite' charged attack spam. His CA has a 50% chance not to consume a stack, so on average he can do twice as many CA.
Itto's C6 is laughably better than ganyu's C6.
4 piece Husk of Opulent Dreams is his bis. DEF sands, Geo DMG goblet, CRIT circlet. Substats: CRIT > DEF, ER.
Redhorn is his clear bis. For 4 stars, the Whiteblind is his best option for damage and is competitive with the other 5 star claymores (except Redhorn). However the Sacrificial Greatsword is also good for energy and accumulating stacks with Ushi (though the Sacrificial Greatsword will not do as much damage as the Whiteblind).
The interesting thing about Itto is that you can give him a Level 1 weapon (even a Debate Club), and he will still do reasonable damage if you level all his other artifacts and talents. This is because his DEF to ATK conversion, which makes the base ATK on the weapon not as important as for other hypercarries in the game. This is why running Sacrificial Greatsword for the extra ER and charges on Ushi can sometimes lead to smoother and even faster clears than running a Whiteblind while struggling with ER.
Itto's best teams are usually mono Geo teams with a healer (to fight the rifthounds). Itto's best teams usually always have Gorou. Itto-Gorou-Albedo is a core that has the most synergy since all 3 units have DEF scaling, with Gorou buffing both Itto and Albedo and Albedo providing off field dps to Itto. The last slot is flex, usually someone like Diona or Bennett who can heal.
Itto can be paired with Zhongli too. However they do not really have the best synergy since Itto scales with DEF while Zhongli scales with HP. Zhongli is usually more valuable on say a Xiao team. Itto already gets -20% Geo RES shred from Geo resonance with Itto-Gorou. Based on the way Gorou works, the team only really needs a max of 3 Geo characters. And Albedo synergizes better with Itto-Gorou than Zhongli does.
Raiden Shogun can work for ER generation, but usually when Itto is ran in Geo teams he generates enough energy already, so Raiden becomes a luxury. Kokomi can also work as a healer slot. Mona can provide extra DMG Bonus to Itto. Dehya, Yae, Beidou can provide sub-dps to Itto. Even Nahida can work (as the crystallize reaction can proc the E).
Anemo characters like Venti can help complement Itto's lack of large planar AoE. First, Itto's vertical AoE is quite good and similar to Raiden's. His CA can even hit oceanid birds. Second, Itto does have decent local planar AoE (within a horizontal plane). It is bigger than Hu Tao's, but it is not as large as some of the top AoE characters.
As with all DPS that rely on their Normal Attack, Childe can help boost the level of Itto's NA by 1.
We assume 4 piece Husk with a Geo DMG goblet and max of 4 stacks (giving 24% Geo DMG Bonus) for a total of \( 46.6+24 = 70.6 \)% Geo DMG Bonus. We assume that he can do his 6-hit (5 stacks + final slash) special CA combo twice and do his 4-hit NA string once within 11 seconds (possible to get the full stacks if using Ushi as well). The burst has an 18s cd. We assume Itto has Lv. 10 NA. The MV is $$z = (2*(180.2*5+377.4) + 720.45)*1.706/18 = 310.61$$
Now Itto's burst gives an ATK buff based on his DEF. We assume a typical ATK scaling character has around 2000 ATK. Now with his Q Itto can reach around 4000 ATK with an average build (when paired with Gorou). Unlike Hu Tao, Itto does not really lose anything when paired with Bennett. But more importantly, Itto does not have a cap on his ATK buff (whereas Hu Tao's E buff cannot exceed 4 times her base atk). Say Bennett gives 1000 ATK so Itto reaches 5000 ATK while other characters reach 3000 ATK. So with Bennett, Itto will have ~1.67 times more attack than a typical character.
So we multiply the above MV by the ATK scaling $$x_q = 5z/3 = 517.68$$ And this does not factor in his E damage (the cow that hits like a truck). The E has around 500% scaling and a 10s cd. (If a sacrificial claymnore is used, the E can sometimes be used twice.) So Itto is another example of a character that has high dps without need for reactions.
At lv10 his E has 553% scaling. It has a 10s cd. Suppose Itto uses it twice during his 11s burst, so it benefits from the atk buff. The dps is $$x_e = 553*1.706*(5/3)/10 = 157.2$$
The total dps is $$x = x_e + x_q = 674.88$$ This assumes 5k atk. If it is say 4500 atk then this is scaled to $$4500x/5000 = 607.392$$
With his C1, after using his Q, he gains 2 stacks and will generate 3 stacks over the next 1.5s. This makes it easier to gain stacks and changes the combo sequence somewhat. In effect, his first CA combo string can technically go to 9 hits (6+3 from the 3 regenerated over time). Afterwards suppose he does his 4-hit NA string once and then another 6-hit CA combo. The MV is $$z_1 = (180.2*(5+8) + 377.4*2 + 720.45)*1.706/18 = 361.85$$ This is about an 1.165 times increase in damage, which is quite respectable. By comparison, Morax's shield (-20% RES) gives a similar increase in damage (around 16-17% in most situations).
If using Ushi right after burst, Ushi can accumulate stacks. The CA combo can then sometimes hit 10 consecutive strikes. When Ushi takes damage it gives 1 stack (once every 2s). Once Ushi's HP reaches 0, it gives 1 stack. Theoretically, during Itto's 11s burst, Ushi can give say around 6 stacks if it continuously takes damage and survives that long. But if we were to build a model on Ushi stack accumulation, that would depend more on the environment and surrounding enemies.
We now consider the effect of C2, which can reduce the burst cd by up to 4.5s (if there are 3 geo characters on the team). This can increase dps to $$x_2 = 18x/13.5 = 690.24$$ which is a 33% increse in damage, comparable to a TTDS buff.
By reducing cd to 13.5s, since the burst lasts 11s, C2 effectively allows near 100% uptime for Itto. There is only 2.5s of downtime, during which other characters' abilities can be used.
As for C6, it gives +70% crit dmg to CA. This is good for low-investment builds. Say one only has the base 50% crit dmg. Then this buff will give at most \(2.2/1.5 \approx 1.47) times more damage. But usually one starts with more than 50% crit dmg with artifacts. Itto can also get lots of crit dmg from the stonethresher and Gorou C6, so the +70% crit buff will give diminishing returns for crit. For simplicity, we will ignore the effects of crit buffs for now. Other characters can get crit through other means as well, such as Sara, crit weapons, etc.
Instead we focus on the other buff provided by C6, the 'infinite' CA. There is a 50% chance to not consume a stack. On average, he can do twice as many CA. So if before he had 5 CA + final slash, he can now do on average 10 CA + final slash (11 hit combo). The advantage of the CA is their increased ATK SPD compared to normal attacks. We assume that the new combo looks something like: 11 hit combo + 5 NA + 11 hit combo. The MV is $$z_6 = (2*(180.2*10+377.4) + 720.45)*1.706/13.5 = 641.87$$ We then scale this by the ATK buff $$x_6 = 5z_6/3 = 1069.78$$ We will assume that a C6 Itto has around the same ATK as a C0 Itto (though C5 does give some extra scaling to the q, it may not change ATK by that much).
Even in a really wild and optimal scenario, where Itto never runs out of stacks, does say 30 CA in a row nonstop, and has say 5000 ATK (so 6000/3000=2 times more ATK than others), the dps is $$2*(180.2*30+377.4)*1.706/13.5 = 1461.7$$ There is not much difference from 1000. It will be within this range of dps. Anyways C6 Itto is already stronger than all C0 characters. Compared to C0, a C6 Itto might provide 2 times more dps.
Itto's main strength is his combination of high ATK and high DEF (literally turning defense into offense), making him able to do lots of damage, while being nearly invincible at the same time (enemy attacks basically tickle him). He is a literal tank and superman.
One of his passives increases his CA by 35% of his DEF. So if Itto has 5000 ATK and say 3000 DEF, the CA scaling will be increase to $$(5000*180.2+35(3000))/5000 = 180.2 + 35(3/5) = 201.2$$ whereas the final slash scaling will increase to $$(5000*377.4+35(3000))/5000 = 377.4 + 35(3/5) = 398.4$$ For C0, there are say 12 hits of his CA so the total increase in dps is $$x_b = 12*35(3/5)*1.706/18 = 23.884$$ which is more or less negligible. This only affects the ca, not the na. The total C0 dps (with Bennett) is then $$x_q + x_e + x_b = 157.2+517+23.8 = 698$$
For C6, if there are say 22 hits of his CA, the increase from this passive is $$22*35(3/5)*1.706/13.5 = 58.38$$ Since Itto already gets so much ATK from his Q, the additive damage increase does not have as much an effect as for other characters.
The best way to increase Itto's dps is by giving him more DMG Bonus (the multiplicative one). Chances are Itto will have super high ATK, DEF, and CRIT. The only thing he needs to boost his dps even further is DMG Bonus. Mona is one of the best ways to do that (note Kazuha cannot 'swirl Geo').
Most of the above calculations hinge on the assumption that Itto is paired with Bennett. But one of Itto's strengths is that he does not need Bennett. Moreover, in most cases, Itto will not be paired with Bennett as Bennett is more valuable in other teams. So here Itto's dps is computed without Bennett for a more typical/realistic outlook.
Itto without Bennett can reach say 4k atk, while other characters with Bennett can reach 3k atk. So Itto's atk is 4/3 times more than others. The damage during his q is then $$x_q = 4z/3 = 414.1$$ while his e damage is $$x_e = 553*1.706*(4/3)/10 = 125.8$$ Total dps is then $$x_0 = x_e + x_q = 539.9$$