Five Island all the way. As Went said, the Rocket Warehouse plot was really interesting, and yes, the G/S references were fabulous. 8D;
The only other Island that comes even slightly close would be Four Island, because I absolutely loved the cave there, and as Loreli is my favorite Kanto Elite Four member, her bit of plot was quite interesting. As well as the fact that you can breed at the Day Care. Many an hour was spent hatching Delibirds.