So many I'd want to use!
4. Camilla/Camille (can't decide! I prefer Camilla as a FN, but Camille as a MN)
5. Cora/Cordelia (again, they're too close to decide! I adore both.)
4. Cillian/Ciaran (tied! They're both Irish/Gaelic, both sound sort-of-similar, and I really love them both!)
Charles and Calvin get honorable mention for boys, and Cassia, Callista, Calliope, Carolina, Catherine/Cate, and Kaia are runners-up for girls.