well Opera was not much of a genre when Bach was around. The genre of the concerto is far different from they that Mozart knew. The Sonata is something the Bach did not know. They symphony was not thought of when Bach was around, thus Bach could not have written in some genres that Mozart did.
Bach is the undisputed master of every form he touched, including:
concerti grossi - Sure they aren't concertos in the normal sense of the term, but lets not forget that he was the undisputed master of this form
All sacred music - This is in itself a massive category, and the masses in particular are crowning achievements of western art. The Mass in B Minor is, more often than any other work, considered the greatest single composition in the history of music, across all forms, time periods, and genres. The only other single work that might compete is if you count the entire Ring cycle as one composition.
keyboard music - Bach's compositions for organ and harpsichord surpass any other composers contribution to any keyboard literature. In fact, Bach did more with the organ and harpsichord than the best 5 piano composers did with piano, combined. I mean, the well-tempered clavier alone would put him in the ranks of beethoven, mozart, chopin, liszt, and other masters of keyboard instruments. When you add to that the goldberg variations, the fugues, the organ work... I mean just his keyboard work alone would suffice to cement his status as at least a tie with Mozart and Beethoven.
sonatas and solo non-keyboard work - I have no idea where you get the notion that Bach had no sonatas... he had many. His solo music in general was, again, staggering. The sonatas and partitas for solo violin are the greatest composition for solo violin, by far. The cello suites are the greatest composition for solo cello, by far.
Lets not forget that Mozart is not the master of all he touched. In opera, for instance, most would put Wagner and/or Verdi as at least the peers if not the superiors of Mozart.
The fact that you are not even considering the fact that no one had composed at the scale that Mozart did. No one could ever match the amount of work he composed in such a short period. He wrote over 600 works in a time period of nearly 36 years, now that is massive.
The Bach catalog has 1127 entries, and most music scholars think that a large number of Bach's works may be lost (some go so far as to say perhaps as much as half of his compositions).
He composed these works over 50 years. Thats 17 works per year for Mozart, and 22 works per year for Bach, even on the outrageous assumption that none of his works have been lost. A more realistic figure (though still probably underestimating things) would put Bach at about 30 works per year.
Heck his mind was built for music, since when he was in Rome, he was listening to a Mass, he was able to write down all the music he listened to, just from ear. Naturally he got told off for doing that, but that is the talent that he had. Mozart was composing at the age of 5, which shows the talent he had.
Stories of prodigies and virtuosos have a certain romantic flair, but they do not a great composer make. We choose Bach's solo violin work over Paganini's any day of the week.
Besides, everybody knows that Mendelssohn was the greater prodigy, in the sense of composing mature, permanent works at the youngest age.
x 1000
I'm not comparing him to Bach or Beethoven, I'm just saying.
