Seconded Madame Butterfly! Puccini is the best.

I think a little bit of exploration as far as what classical eras and nationalities of composers you like will go a long way. Mozart is my one of my favorite Classical composers and also writes amazing operas; honestly the movie Amadeus could be a great primer and features opera as a plot device, with great studio recordings and execution.

I would try out some Wagner(Tristan und Isolde), Mozart (Don Giovanni), Puccini (Turandot). I love French composers but have never been much into French or English opera. In my opinion, overtures and ensemble movements over arias are the real audio ticket, but a track like Nessun Dorma from Turandot could very well be a track I might use to test out some speakers for a great mix.