Of those currently on at ENO or ROH I would choose Mozart – Don Giovanni.
If you can find it coming up anywhere then Britten – Peter Grimes. My absolute favourite opera. The best of the 20th century, or very possibly EVER. Just my opinion of course.
I’m on the lookout for a live performance of Janacek’s Cunning Little Vixen. I reckon that’s worth a go.
You could try getting hold of opera DVDs at your library to try out in the safety of your own home.
Oh and I’m with you on musicals. Normally banal trash with a scattering of good songs. Mind you there is West Side Story!!!!!! Musical? Opera? Who cares, it’s brilliant.