X/Y shocked me, because I had found every Pokemon game's OST to be forgettable since R/S/E, but X/Y's OST was suddenly great. Whereas Generation V had maybe two or three memorable themes for me, X/Y had many.
My most favorite themes are probably Route 4, the Power Plant, and Victory Road. However, there are many memorable themes - including Lumiose City (at first I didn't like it all of that much, but it has grown on me), Parfum Palace (such faux Baroque lol), pretty much all of the Route themes, the Champion's theme, Laverre City, Anistar City, etc.