UID-- jk it's not a contest anymore
Intro
Yanfei is a brand new 4-star Pyro character released in Genshin Impact 1.5. Her existence may not be widely known as she's only mentioned a few times, e.g. in Ningguang's and Keqing's Voicelines. This so-called Pyro Klee doesn't only rival Ningguang in terms of the law, but she also has a similar playstyle with the Tianquan.
Here, I will be testing a few weapons and artifact sets to determine the best build for DPS Yanfei.
Conclusion (TL;DR)
Best Weapon for Melt comp:
1. Lost Prayer to the Sacred Winds, Memory of Dust (With shielder)
2. Skyward Atlas
3. The Widsith
4. Solar Pearl
5. Mappa Mare
6. Eye of Perception
7. Sacrificial Fragments
8. Favonius Codex
Best Weapon for No-Reaction comp:
1. Lost Prayer to the Sacred Winds, Memory of Dust (With shielder)
2. Skyward Atlas
3. The Widsith
4. Solar Pearl
5. Eye of Perception
6. Mappa Mare
7. Favonius Codex
8. Sacrificial Fragments
Best Artifacts for Melt comp:
1. 4-Crimson Witch
2. 4-Retracting Bolide (Only if you have 100% shield uptime)
3. 4-Wanderer's Troupe
4. 2-Crimson Witch + 2-Gladiator's Finale
Best Artifacts for No-Reaction comp:
1. 4-Lavawalker
2. 4-Retracting Bolide
3. 2-Crimson Witch + 2-Gladiator's Finale
4. 4-Crimson Witch
5. 4-Wanderer's Troupe
Choosing the Weapon
8 different weapons will be tested on Yanfei:
1. Eye of Perception
• Passive will be considered.
The passive will be simulated to proc twice in a 54 seconds simulation.
2. Favonius Codex
• Passive will not be considered.
3. Mappa Mare
• +16% Elemental Damage bonus is always active.
4. Sacrificial Fragments
• Passive will be considered.
5. Solar Pearl
• +20% Normal Attack, Skill, and Burst Damage bonus are always active (No bonus for Charged Attack).
6. The Widsith
• Passive will not be considered in the tables, but I will explain the passive further.
7. Skyward Atlas
• Passive will be considered.
The damage passive will be simulated to proc once and hit 5 times in a 54 seconds simulation.
8. Lost Prayer to the Sacred Winds
• The Passive considered will be +16% Elemental Damage.
Choosing the Artifact
The tested main stat is Attack/Pyro/Crit. 5 Artifacts will be tested:
1. 4-Wanderer's Troupe
2. 4-Crimson Witch
3. 2-Crimson Witch + 2-Gladiator's Finale
4. 4-Retracting Bolide
• Shield is assumed to be always active.
5. 4-Lavawalker
• Enemies are assumed to always be affected by Pyro.
Final Stat
Note: The Widsith is shown without any buff in this table.
Simulation and assumptions
1. Yanfei is lv90, talent lv6, C0.
2. The enemy is one lv80 Hilichurl.
3. Critical Rate will be simulated at 60%.
• Both Crit and Non-Crit hits will be calculated.
Afterward, I will simulate the average of 40% Non-Crit and 60% Crit. This is done to give Crit build less edge.
4. Yanfei's first passive will not be considered. However, the second passive will be considered.
• Yanfei's second passive won't proc in the 40% Crit calculation.
5. Yanfei's attack sequences:
• Normal Sequence: Normal Attack x 3 + Charged x 1 (~3 seconds)
• Under-burst sequence: Normal Attack x 2 + Charged x 1 (~2 seconds)
6. Because Yanfei's Burst focuses more on its Charged Attack buff than damage, I will not be doing a DPS method in the calculation. Instead, I will run a ~54 seconds simulation that goes like this:
Burst + charged* (1)
Skill + charged* (2)
2 hits + 1 charged* x 5 (10)
Skill + charged* (2)
3 hits + 1 charged x 3 (9)
Skill + charged (2)
3 hits + 1 charged x 3 (9)
Skill + charged (2)
3 hits + 1 charged x 3 (9)
Skill + charged (2)
3 hits + 1 charged x 2 (6)
So that's:
Normal Seq. = 11
Charged = 14
Skill = 5
Burst = 1
Burst Seq. = 5
Charged* = 8
While for Favonius Codex, it will look like this:
Burst + charged* (1)
Skill + charged* (2)
2 hits + 1 charged* x 5 (10)
Skill + charged* (2)
3 hits + 1 charged x 3 (9)
Skill + charged (2)
2 hits (1)
Burst + charged* (1)
2 hits + 1 charged* x 3 (6)
Skill + charged* (2)
2 hits + 1 charged* x 3 (6)
3 hits + 1 charged x 1 (3)
Skill + charged (2)
3 hits + 2 charged x 2 (6)
2 hits (1)
Normal Seq. = 6
Charged = 9
Skill = 5
Burst = 2
Burst Seq. = 11
Charged* = 16
Note: The numbers in the bracket represent durations. E.g. 3 hits + 1 charged takes 3 seconds to do.
Charged* = Charged Attack under the buff off Yanfei's burst.
Normal Seq. = 3 Normal Attack hits
Burst Seq. = 2 Normal Attack hits
Why 54 seconds? Because when I simulate 1.5 Energy regeneration per second, it takes 53.3333 seconds to regain her Burst back.
Artifact and Weapon Test
Elemental Mastery bonus estimation:
Damage Overview
Non-Crit damage overview:
Crit damage overview:
Simulation Result
Non-Crit simulation:
Crit simulation:
Note: Passive 2's damage also Crit 60% of the time, and some of them are under the damage bonus from Yanfei's Elemental Burst.
Results Overview
After summing the Non-Crit total and Crit total, the sum is divided by 10 to obtain the "Average value".
Here you can see that 4-Wanderer's Troupe places last in a non-reaction comp. Quite surprising considering Wanderer's Troupe seems to have the potential to be Yanfei's best-in-slot set.
In Melt comp, 4-Wanderer's Troupe also fails to take the top spot, placing #3 on the list.
Note: Melt comp calculation is a highly theoretical number where I assume melt reaction happens 100% of the time.
So, we obtain the following conclusion:
Best Artifacts for Melt comp:
1. 4-Crimson Witch
2. 4-Retracting Bolide (Only if you have 100% shield uptime)
3. 4-Wanderer's Troupe
4. 2-Crimson Witch + 2-Gladiator's Finale
Best Artifacts for No-Reaction comp:
1. 4-Lavawalker
2. 4-Retracting Bolide
3. 2-Crimson Witch + 2-Gladiator's Finale
4. 4-Crimson Witch
5. 4-Wanderer's Troupe
Next, I will take all the numbers from 4-Crimson Witch, and sort them. Now we have:
You can see that Lost Prayer takes the top spot in both cases. It is important to note that The Widsith here is shown without any buff.
I tried calculating a 10 seconds buff + 20 second without buff, and The Widsith places higher than Solar Pearl (The buff tested was +48% Elemental Damage.)
If you're wondering why Skyward Atlas falls hard in Melt comp, it's because the weapon's passive does not get amplified in Melt calculation.
You can also see that Sacrificial is a "meh" choice for Yanfei. Furthermore, Favonius Codex's Energy Recharge fails to increase Yanfei's performance.
Edit: I tested Memory of Dust (based on Vasir's request), and found that at full stack with shield (+40% attack), Memory of Dust perform better than Lost Prayer. But at full stack without shield (+20% attack), it performs less than Skyward Atlas. Since the weapon is situational, I put Memory of Dust and Lost Prayer in the same ranking (#1).
Personal Opinion
The first time I used Yanfei, I wasn't really impressed. But after trying her again for the sake of this analysis, I found that her flow of attack is really pleasant.
It's very easy to maintain the 1-2-3 charged pattern, and her hits are really smooth, unlike Ningguang's that's clunky af (even after her charged attack is "fixed"). Moreover, her stamina consumption is not bad at all. Especially if you have her at C1, then you won't feel like you're using stamina.
Although Crimson Witch is technically better than Wanderer for Yanfei, the difference is quite abysmal. So if you can do both sets, then I'd recommend choosing the set with the better substat.
As for the weapon, Mappa Mare is a viable choice for Yanfei. On the other hand, Sacrificial seems to be a poor one. So good news for F2P players.
I think that Yanfei can be a better DPS than Ningguang, but she will be very high maintenance.
Unlike Ningguang that can solo anything and fit in any team, Yanfei will need a team built around her. Unless you have C4 Yanfei, she will also be squishy af, which means more DPS loss while supporting her.
Other than that, Yanfei's damage is also mediocre without reaction, unlike Ningguang that goes raw on any foe. So unless you're willing to invest deeply in Yanfei, I won't suggest building her as a DPS.
Random idea from my boyfriend:
If you have Yanfei at C4, you can make her a shielder. HP/HP/HP with lots of ER substat for 100% Burst uptime. You can have ~10k Pyro shield that last for 15 seconds, and can be casted every 20 seconds. 
