Beethoven and Mozart. I could happily list a lot of lesser composers whose work I also love, but there are good reasons Mozart and Beethoven are so well known, and the main reason is their music is so overwhelmingly good.
As a side note, apart from the masterful Pathetique, I have a real problem with Tchaikovsky. Doesn't he ever know how to finish his music ? The otherwise good 5th Symphony is a good example, where he can't decide how to finish, so he just appends one ending after another. This tendency becomes ridiculous with the Capriccio Italien, where it feels like almost half of the music is just various attempts at ending the piece.