Phew, back on GSFs after a years-long hiatus...
As a musician myself, I've an incredible amount of respect/admiration for Regina Spektor. Her voice and piano riffs are absolutely mesmerizing... need I even mention, for instance, the inarguably commendable fact that she recorded her second album in a single day, in one take.
So many levels on which one can appreciate her.
EDIT: I hate spotlighting individual songs, but since the title asked... I've always considered Christina Aguilera's "Fighter" to be an indisputable classic. Won't go beyond that; too intractable for me!