well, if you ask me, i really can't decide. They have unique characters. IMO, Kozue Amano-sensei is a prodigy.But if i really must pick one, my favorite character is Alice, the "Orange Princess" because she's really a prodigy.
My favorite Aria character is a toss up between Akari and Akatsuki the Salamander. Sure Akari is great and she gets plenty of screen time and development, but Akatsuki is only a supporting character and gets significantly less screen time, yet he easily holds his own when he is on screen. I'm halfway through Origination but i hope to get some background on him soon.
I tried doing this once...didn't work out for me haha. I just find the whole cast brilliant and am unable to choose just one. Aria is just a bundle of...medicine in this hectic society imo, shows us that sometimes, we have to slow down (I would say Akari teaches us that the most ^-^).
Definitely Akari, though I love Aika as well. I think Akari's optimistic personality paired with her ability to find value in anything fits the themes of the series perfectly, and she's a very endearing character.
Every character has their own unique trait and development progress, however Alice stands out the most from my oberservation.
While Akari has a tendency to enjoy almost everything she catch sight of, Alice on the hand seems to get easily bothered by what she see. She always pays attention to her surroundings, keeps an eye on everything, including her seniors undine, like how she thought Akari got bullied by Akatsuki in E3S1, how she wonders if Alicia ever gone mad with Akari, or how she's worried about the future of the trio after her promotion to Prima Undine.
Alice is also talented in rowing gondola, but her lack of communication skill supressed her from reaching the fullest state. Throughout the series we can see her gradual development, mostly thanks to Akari who enlighted her to become more cheerful. The result is at E9S3 we get to see the full-flegded Alice, skilled in every aspect, her singing voice is also beautiful and touching, she completely took my heart away at that scene.
Sometimes she can act childish (she's only 14 year-old anyway), but those childish act is realistically portrayed, i bet most of us were like that at that age. While she's the youngest, i find her the best to be able to keep her composure among the trio, which is cute as she always act like she's grown-up but in fact she's got a long way to go. Her self-establish rule is also one of her unique trait, which i can pretty much relate, living with a code is cool eh.
Her relationship with Athena is the best imo, senior and junior don't always get along, but these 2 are super cute and hillarious sometimes. While Alice seems to always push Athena away because of her over-caring trait, she's in fact doubtful about Athena's motive, as she usually is by herself so she doesn't trust other much, she's the type to believe that everything comes at a cost. But when the 2 reconcile at E13S2, it was damn touching as Alice finally realize Athena's goodwill.