The Goldberg Variations. I can't believe that I never listened (and paid attention) to a full performance of it before yesterday. My appreciation for Bach has grown over time, and not just because I force myself to play his music. Well, maybe a little, but I realize that his music is extremely well constructed, and it's such a satisfying feeling hearing it all come together in a colossal masterpiece.
I've been obsessed with the Romantics ever since I discovered classical music, and now I realize that great music doesn't have to be coated in agonizing emotion (or varying degrees thereof).