Quick (subjective) take on what draws me to opera
Operas that appeal to me strongest are those in which the music for individual roles seems to convey the most intensely intimate and well-rounded characterization. When I feel that the music for an opera (however clunky the libretto) has given me as deep an emotional knowledge of the psychological core of each character as I have of my closest friends, that opera then gets slotted at or close to the top, and stays there. I also note that the vividness of musical characterization tends to trump, for me, considerations like tautness of narrative, dramatic structure, etc. If time stands still while the music is seeing inside a character, I'm perfectly happy to stay there as long as the composer likes.
The composers whose operas usually give me an intimate feeling for their characters are Haydn, Mozart, Beethoven, Donizetti, Berlioz, Wagner, Tchaikovsky and Janacek. So these eight would be in my top slot.
Just the tiniest step down from those are the composers whose operas often, rather than usually, give me this intimacy: Monteverdi, Purcell, Handel, Bach (I view the Passions as operas, and the most intimate feelings are expressed in their music), Rameau, Gluck, Cherubini, Rossini, Weber, Bellini, Verdi, Mussorgsky, Boito, Leoncavallo, Puccini and Richard Strauss. To have as many as sixteen in slot Two may be a bit cumbersome, but I freely admit that these two tiers are guided more by my feelings than by logic.
-- Geoffrey Riggs