near the big gardens (east of old city) the Bar Rosanero (google map Palermo and enter it in the search box to get info). great atmosphere, good pastries in general.
Scimone was great but a little bit west of the main action. Wonderful ricotta-filled pastries.
in the new city, the via principe di belmonte (the fancy walking street) has some of the fancier pastry shops but I don't remember them being the best.