Yes, well, Wagner, real important and all that, for sure. Highly influential. His experiments in tonality, chromaticism, and the unification of forms all obviously a big deal. But a) he wrote Opera only (or almost only), and b) the classical forms and formats that he might appear to have transcended in fact lived on past him, while the format his championed did not. Corigliano still writes symphonies, Carter and Feldman wrote string quartets, and many many modern masters have written piano sonatas. Many of all of these still employ classical forms such as sonata allegro, rondo, fugue, and many are rich with counterpoint. But where are the modernists who write anything really Wagnerian? I don't deny the influence of course, but it is easy to overstate it.
Reason (a) above -- writing in either one or only a handful of formats -- similarly for me disqualifies composers like Bruckner, Mahler, Chopin, Verdi, and some others from the list. The composers I listed above mastered all the formats and instruments, and were prolific as well.
Learsfool says many interesting things, most of them very reasonable. but being a horn player no more requires or justifies selecting Mahler than, e.g., being a classical guitarist would require or justify picking Fernando Sor. I had 2 good friends in college, one a tuba player and the other a violinist. The tuba player knew and loved Mahler, Dvorak, Bruckner, Tchaikovsky, Mussorgsky-Ravel, Holst, Prokofiev, Strauss. The violinist knew and loved music, period.
