The Greatest Ten According to Tommasini
You know for sure the boutade about Beethoven (the greatest) and Mozart (the unique). This speaks actually about the futility of any classification. Nietzsche was able to compare the two titans and to say some profound words about them, but he was himself a titan.
On the other hand, trying to determine who were the greatest ones is definitely subjective while not useless. Because you need to bring some good reasons to support your choices. And sometimes these reasons are really good, even if your classification remains subjective.
The musical critic Anthony Tommasini brings such a classification in today's NY Times: his top ten composers of all times, from Bach to Bartók. You can read his opinion at