It often strikes me as odd that I work IN opera, since I wasn't brought up on it. The only classical music around our house during my childhood was my dad's cherished copy of Leonard Bernstein conducting "Rhapsody in Blue" and "An American in Paris." :-)
Anyhow, since discovering opera when I was around 17, I've been partial to Puccini [ooh, unintentional alliteration!] It all started when my AP music class listened to a recording of "La Boheme" -- the sweeping and bittersweet melodies really hooked me; must have something to do with my being a hopeless romantic all my life. :-). I think I've seen all of Puccini's works, either live or on tape/DVD. "Girl of the Golden West" is the only one that just does nothing for me, because the American old west is at odds with lush Italian music. Having done Boheme, Butterfly, and Turandot onstage, I have to say that Turandot is the most exiciting, from the animalistic bloodlust of the opening to the thunderous choral reprise of "Nessun Dorma" at the end. Bliss!
However, all those years ago I was almost immediately drawn to American works, and I've become quite a champion of them. When people snobbisly disdain American music, I want to kick them in the shins -- all those Eurocentric attitudes are a little annoying. And I'm a firm believer that American singers should learn at least SOME works in English since it's their native language. Trust me, there's a lot of resistance to this, as if singers are almost ashamed of speaking English. My position is that since the vast majority of European artists sing primarily in their native languages, why shouldn't we do the same?
Anyhow, I'm very partial to the operas of Carlisle Floyd, Aaron Copeland, Gian Carlo Menotti, and of course Leonard Bernstein [the finale of Candide is bar none the most astounding thing ever written -- it's just overwhelming in growing from pensive softness to passion overload]. Favorite works include Porgy & Bess, The Tender Land, and The Passion of Jonathan Wade [I still don't understand why it wasn't a bigger hit -- I did it at Houston and San Diego, and found it magnificent].
But though I like American music [operatic and non-vocal], I enjoy numerous European 20th century composers. I like Poulenc's "DIalogues of the Carmelites" a lot [it's ending is both shocking and profoundly touching], and am quite passionate about most of Kurt Weill's works. Oh, and throughout my career I've done every one of the Gilbert & Sullivan works [including the final two, which aren't very good] -- in fact, my first professional stage job was in "Pirates of Penzance."
I also adore some works from the American theatre. There are some pieces that have sizable scores and thereby tell their story primarily through music -- which really is the definition of opera. I'm a die-hard fan of Sondheim's "Sweeney Todd" -- please keep any groaning to yourself! :-) Weill's "Street Scene" was done on Broadway but is considered by all to be an opera; he was quite a chameleon, as his early German stuff sounds rather different than the "American" tone he adopted/developed after emigrating to the U.S. [glad he escaped Hitler!] Oh, and Frank Loesser wrote an utterly charming musical called "Most Happy Fella" which has relatively little dialogue; indeed, since the leading male role Tony is an Italian grape farmer in California's Napa Valley, Loesser wrote a ton of quasi Italian opera music for him to sing. [It's odd that I adore this Loesser work but generally dislike his "Guys and Dolls."] And more recently, I thought "Ragtime" would do very well in an opera house -- it's an immensely sweeping score that needs some serious vocal fortitude, as well as a cast/ensemble of Wagnerian proportions.
Finally, I liked your final sentence about loving "Butterfly" but hating the story. The nice thing about being gay is I can't be a misogynist, and I've never understood the peculiar appeal for seeing women being put upon. And it's rather ludicrous how many women kill themselves at the end of an opera because they just can't go on without their man. :-)