From the lv90 bows that I currently have to test with 3 of them made sense for me to try, Stringless, Blackcliff, and Mouuns Moon. Due to Sara's attack buff from her elemental skill on how it scales with base attack I do recommend to use the highest base attack you can get just like you would with Bennett. If you had to pick from 4 star bows the highest base attack you can get is 565 and both Blackcliff and Mouun's Moon have that. Stringless falls short at 510 so I normally would not recommend that for Sara. Sacrificial Bow has 565 too but damage wise it will be lower than blackcliff. The ER can be nice though.
From the damage shown here r5 Mouun's Moon is a very powerful 4 star bow. With a party that has 330 total energy (80 x 3 and a 90) the extra burst damage from this bow is 79.2%. Sorry I did not have any 5 star bows to test but from what I saw elsewhere r5 Mouun's Moon on burst damage alone stands up with R5 5 star bows so this has me very excited.
