It's difficult for me to select any given piece for Aria, since different aspects of her character often conflict harshly with one another.
In the end, though, I think I have to go with "Dream Island Obsessional Park,"
the theme to the anime, "Paranoia Agent."
At heart, Aria genuinely believes neither in her own existence nor in anybody else's.
I'd post the full version, only you really also need both the visuals and the subtitles for this one.