The first that come to mind are likely Wolfgang Amadeus Mozart and Ludwig van Beethoven.
But I basically listen to many composers, and some of my favorite works are from neither of those two.
Now I do remember a classical concert where the orchestra suddenly after many works from classical times, started to play "Hedwig's Theme", which may to most people be more commonly known as the tune that starts all Harry Potter movies, and I gotta say: Why not? The tune does definitely fit among the works of the older masters.
So in the end I don't have a real favorite. It all comes down to my mood at given times. I do appreciate some classic works every now and then, though!