For girls, I lean toward S names. While I have the most A names on my list, that's only because A is the most common letter to begin with. I think I have the most S names proportionally.
Sabrina,
Saira,
Sakura,
Samantha,
Saoirse,
Sasha,
Saskia,
Scarlet,
Selina,
Serafina,
Skylar,
Sonia,
Sophia,
Soraya,
Stephanie,
Summer
For boys, it seems to be C, both proportionally and literally.
Caleb,
Callisto,
Cameron, Carmello,
Carmine,
Carter,
Casper,
Cassian,
Cillian,
Chandler,
Chase,
Cole,
Collin,
Cyrus