I didn't like Sansa at first; I thought she was too bratty and preferred Arya instead. As there series progressed though, it wound up the other way; didn't like Arya so much, but loved Sansa.

As marketchippie mentioned, Sansa has huge potential to make some serious power plays in a Cersei-like manner. That she has the chance to observe Petyr (despite how skeevy he is) really makes me want her to become just a BAMF at playing politics.