I loved Olivine City definitely nothing to do with my crush on Jasmine, then, which, I've only recently discovered, is the same as the tune for Violet City?
I'm pretty certain I loved the Bike theme in GSC, but also love the original RBY Gym theme (the tune that plays inside the gym).
I'm sure more favourites will come to me as I read more of this thread!
EDIT: How could I forget, I loved Buena's Password Theme!!!