I think they're all lovely but my favourite is Cecily.
Cecilia makes me think of the Simon & Garfunkel song (which came out long before I was born!) and Celia of the Jonathan Swift poem.
Celia sounds like someone slightly older to me - not sure of the data but I know more Celias in their thirties and up, while the Cecilies and Cecilias I know tend to be younger.