Thanks for the input!

Bach & Mozart handed down the rules of counterpoint and voice leading. One could argue that Beethoven was standing on their shoulders, as well as his own teacher, Haydn's.

This is a point that I have often seen made. However, I don't think that because you learned something from someone else that necessarily means that your achievements are theirs. Haydn, Mozart, and Bach learned from someone else who learned from someone else. The cycle goes on forever. In order to be concrete, let's call Beethoven's achievements his own. But, fair point.

Mendelssohn wrote fantastic compositions at a much younger age, as a teenager.

This is true, but these compositions did not have a huge cultural impact. Mendelssohn was a good composer, but he was not known to be a huge rule breaker like Beethoven. He innovated, but not on the level that other composers of his time innovated.

Schubert was a far better song writer.

This may be true, but it is due to the fact that Beethoven did not prioritize songs like Schubert did. I will say that Beethoven has songs which are just as innovative and complex as Schubert's, but there is a reason why Schubert was called the king of song. However, there is also a reason why Beethoven was called the king of harmony by the same person. Both men were talented, but I again feel like Beethoven had a larger cultural impact.

Mozart was a far better opera writer

Beethoven was a far better symphony writer. The reasoning is again priorities and focus. Mozart dedicated a lot of his time and effort to opera. Beethoven did the same with his symphonies. However, I will say Fidelio is the first opera with a female protagonist. Like that has anything to do with musical achievement.

Mozart had the best-selling CD of 2016.

That's because there are like 10 people who still actually buy CDs. Though I applaud the market based approach. I will say though that a lot more people know Beethoven's music than Mozart's. For example, here's a list of the 10 most famous pieces (that I just found and probably has no scientific backing). Beethoven is the only composer on the list with 2 pieces.

James Horner's Titanic Soundtrack was the best-selling classical music CD in the last 25 years, which covers much of the time when people have been able to buy music en masse.

This music came out much more recently than Beethoven's. It was also featured in a major motion picture.

Bach and Schubert both composed massive numbers of compositions.

Beethoven had a descent amount. Probably a lot more than James Horner. I again will make the point that Beethoven's music had a larger cultural impact than Bach or Schubert's.

Thanks for the feedback!