Beethoven's strength was in form and drama, building excitement and using time as a leverage, using dynamic contrasts for expression, developing (multiple) themes in unpredictable and complex ways.
If we are looking at just harmony itself in my opinion Beethoven wasn't a master to the same degree as Bach or Mozart, (or Brahms for that matter).
Wagner was very innovative with harmony but over all I don't consider him a master of harmony to the same extent of Brahms, if we are looking at aspects of harmony like counterpoint and how the harmony relates to form.
The greatest masters of harmony of the 20th century in my opinion were Debussy and Ravel.
If we are looking at just harmony itself in my opinion Beethoven wasn't a master to the same degree as Bach or Mozart, (or Brahms for that matter).
Wagner was very innovative with harmony but over all I don't consider him a master of harmony to the same extent of Brahms, if we are looking at aspects of harmony like counterpoint and how the harmony relates to form.
The greatest masters of harmony of the 20th century in my opinion were Debussy and Ravel.