Celia for me.
Don't like Cecily - sounds too lispy and a bit 'wet' (like a limp girly character from a children's book who hates getting her clothes dirty - might just be me though!
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Used to love Cecilia but now find it too fussy and frilly - too many s sounds. Agree that Celia's a bit 'librarian-chic' but see that as a good thing! It's less girly and more streamlined than the others, plus it has a lovely meaning and a Shakespearean namesake. Feels more solid and substantial than the others somehow.