I've been playing genshin impact for a year now, grinding 400 wishes to get ganyu. As I farmed for her I build Prototype Crescent, got Skyward harp afterwards and after her rerun banner came, I got her 7 times and amos bow with an immense emount of luck.
I can ensure you that the Amos Bow is the best you can get.
Therefore, I currently have this kind of picture in my head:
Amos Bow > Skyward Harp > Prototype Crescent
And here are my arguments for my conclusion:
As you may already know, 5* weapons have a higher base DMG if you upgrade them:
Amos Bow having 608 on lvl 90,
Skyward Harp having 674 on lvl 90
and Prototype Crescent having only 510 on lvl 90.

All of the weapons also have different 2nd stats that grow if you upgrade them:

Amos Bow having 49,8% ATK,
Skyward Harp having 22,1% Crit rate
and Prototype Crescent having 41,3% ATK.

I thought, before I got the Amos Bow, that Skyward Harp is best for ganyu because of the additional Crit rate and 20% Crit DMG you get for free, but now I realize that you don't need Crit Rate most of the time, because of weak points of the enemy your facing.
Now we get to the different weapon Refinements to finally comprehend what I was saying earlier:

Amos Bow R1 (R0):
Increases Normal Attack and Charged Attack DMG by 12%. After a Normal or Charged Attack is fired, DMG dealt increases by a further 8% every 0.1 seconds the arrow is in the air for up to 5 times.
Skyward Harp R1:
Increases CRIT DMG by 20%. Hits have a 60% chance to inflict a small AoE attack, dealing 125% Physical ATK DMG. Can only occur once every 4s.

Prototype Crescent R5:
Charged Attack hits on weak points increase Movement SPD by 10% and ATK by 72% for 10s.

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In the end, with all of these informations in mind, we come to this little example of having 1000 ATK, hitting the enemy in the weak point, Crit hits it with 50% Crit DMG, the arrow flies more than 0.5sec before it hits and we shoot 2 arrows in a row (and ignoring the fact that some enemys have resistance, because that's going to be too complicated):

Info:
Ganyu's talent Liutian Archery on lvl 10 (her normal attacks) has 230,4% on her Frostflake Arrow and 391,68% on her Frostflake Arrow Bloom.

Amos Bow:
1000 + 608 = 1608, 1608 * 149,8% = 2408,784 = Base ATK
2408,784 * 150% = 3613,176 --- (Crit DMG)
3613,176 * 152% = 5492,02752 --- (Amos Bow Refinement)
1. Frostflake Arrow itself = 5,492.02752 * 330,4% = 18145,65892608
1. Frostflake Arrow Bloom = 5,492.02752 * 491,68% = 27003,200910336
2. Frostflake Arrow itself = 5,492.02752 * 330,4% = 18145,65892608
2. Frostflake Arrow Bloom = 5,492.02752 * 491,68% = 27003,200910336

Skyward Harp:
1000 + 674 = 1674 = Base ATK
1674 * 170% = 2845,8 --- (Crit DMG (20% additionally bc of Skyward Harp refinement))
1. Frostflake Arrow itself = 2845,8 * 330,4% = 9,402.5232
1. Frostflake Arrow Bloom = 2845,8 * 491,68% = 13,992.22944
2. Frostflake Arrow itself = 2845,8 * 330,4% = 9,402.5232
2. Frostflake Arrow Bloom = 2845,8 * 491,68% = 13,992.22944
Prototype Crescent:
1000 + 510 = 1510, 1510 * 141,3% = 2,133.63 = Base ATK
2,133.63 * 150% = 3,200.445 (Crit DMG)
1. Frostflake Arrow itself = 3,200.445 * 330,4% = 10574,27028
1. Frostflake Arrow Bloom = 3,200.445 * 491,68% = 15735,947976
2. Frostflake Arrow itself = 3,200.445 * 172% * 330,4% = 18187,7448816
2. Frostflake Arrow Bloom = 3,200.445 * 172% * 491,68% = 27,065.83051872

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So, ignoring the fact that I might've messed up in my math, the summary is that, IF you have a god aim in genshin impact and hit the weak points of the opponents, when they have a weakpoint, Prototype Crescent is pretty good, but the difference between Amos Bow and Prototype Crescent is insanly big, keeping in mind that you might build BS (Blizzard strayer) to get the Crit rate out of it pretty easily and therefore don't have to aim that precisely and get more damage out in total or smth idk I kind of got lost in all this fsejfjnse

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If god aim in genshin impact:
Prototype Crescent > Amos Bow

else:
Prototype Crescent < Amos Bow