Gameplay(Outside of Shyvana)
The best kind of gameplay for a champion, IMO, is beauty in simplicity. Sona may be the queen of it, but Soraka and Nami are among the queen's daughters.
Visuals(Outside of Shyvana)
All 3 have visually uncomplicated designs: Sona has a beautiful oriental dress and instrument, Nami is a mermaid with a staff, and Soraka is kinda just pleasant.
Theme(Outside of Shyvana)
Sona's design and theme are clean and easy to get: She's a mute, but has incredible music. Nami is a mermaid with spells and a staff. Soraka is an ex-deity.
Personality(Outside of Shyvana)
Sona's personality is unique: She's not quite manipulative, but she clearly knows her powers and uses them. Nami is still a teenager, excited by adventure and a bit naive, but her playful personality just locks her in my memory. Soraka is clearly a careful and gentle being, preferring to heal the injured than be a warrior.
Lore(Outside of Shyvana)
Sona's lore is somewhat captivating and a bit somber. Nami's lore is pretty much just "find the magical MacGuffin", which is fine by me 'cause Magical MacGuffin is a pretty good incentive to complete your quest. Soraka's is just interesting to read.
Yes, I know I'm generic for choosing the same 3 champs each time, but I can't help it. They're all so well designed that I can't help but pick them. ._.