The rudest bit of opera I've ever listened to comes from Prokofiev's "Lady McBeth of Mtsensk District". You can hear exactly what is going on.
The most insanely difficult aria I've ever listened to (and performed) in opera comes from "Ariadne auf Naxos", the convoluted and extraordinary aria sung by the cool little flirt Zerbinetta.
The most boring opera I've ever listened to is (in my opinion) "Martha", by Flotow. There's only one 'famous' aria in that opera, and the unfortunate heroine has to sing it over... and over... and over again.
The most humane opera I've ever listened to is possibly "Die Entführung aus dem Seraglio" by Mozart. But then everything Mozart composed is pretty close to perfection... and when it's not perfect, it still sounds like it.
And opera is completely off-topic on this thread. Hence it is on-topic.