Being able to relive the Sapphire/Ruby memories...
Pokemon Sapphire was the first game I ever had and I remember the tears when I had to give it away.
Then those tears came back when I heard the Rival Battle track from ORAS. I don't think a game has ever made me cry so much...
So I guess the nostalgic feeling that comes from hearing the soundtrack which I think is the best remade soundtrack out of all the remakes