For Sona:
DJ almost always, but typically Silent Night or Sweetheart if I don't want to use it. I usually only use arcade for other arcade skins.
For Orianna:
Winter Wonder almost always, I love the light colors and recall animation. Heartseeker if playing Orianna support or whenever I'm feeling it. Gothic whenever I'm serious about a match. Despite owning every skin, I usually don't use the others compared to those three.
For Nami:
Typically I use Koi (Have to love the koi in ebb and flow and the flutes during the ult), but use Deep Sea whenever I want to get serious. Occasionally I use River Spirit but that's only if I want to match with my adc's skin.
For Taliyah:
Usually choose Freljord as I adore all of it's effects and Taliyah's recall. I also enjoy the freljordian markings on her arms and her hair. However, I choose default when I want to switch things up or enjoy the pleasing sound effects (I love the q and e).
For Aurelion Sol:
Classic. Always.
Sorry if this was long winded!