If you only ever listen to one opera all the way through, I might recommend Mozart's Don Giovanni.
True, I am biased, as it is one of my favorites, but it is also a treasure trove of importance and significance.
Mozart partnered with Figaro librettist Lorenzo Da Ponte for the second time on Don Giovanni, and the two produced another opera buffa.
So, what's opera buffa? Well, in Giovanni, Leporello is the "basso buffo"... think of an auctioneer in an opera. Opera buffa is simply a comedic opera with a strong bass role (the auctioneer), and generally has fewer acts than the more serious genres of opera.
Yes, Don Giovanni is a comedy, but it is a rather dark one. I mean, dude gets dragged into hell by a reanimated stone statue of a guy he killed.
The overture is quite creepy. It's my favorite opera overture of all, yet we never hear it as a stand-alone piece since it doesn't resolve at its conclusion - it ends in a different key than the one in which it begins (begins in D minor, ends on a V chord in C major, for all you theory nuts out there).
And speaking of keys, check this out. Keys (for instance, C major or C minor) sounded drastically different from each other, before we began tuning pianos in equal-temperament. This is the kind of thing that can either a) keep you up at night trying to comprehend, or b) make your head explode.
Because Mozart and his many predecessors lacked equal-temperament, these different-sounding keys took on special meanings, and were used to symbolize endless amounts of extra-musical significance.
E-flat major was used for nobility or heroics (think Beethoven's 3rd Symphony). G major symbolized pastoral or peasant music. And so on. By writing music in specific keys to symbolize, or even telegraph, meaning beyond that which words convey, Mozart was no different than other composers before (and after) him.
Mozart constantly toys with D major and D minor in Don Giovanni; a hidden musical battle between the celebratory, triumphant D major and the piety and melancholy of D minor.
Christian Schubart wrote a book about it (not the first) in 1806, not long after Don Giovanni's Prague premiere.
In the video, you can watch Gerald Finley sing one of the few arias Don Giovanni himself gets to sing, the quite famous "Champagne Aria."
Be sure to tune in Saturday to a live broadcast from the Metropolitan Opera of Mozart's Don Giovanni at 11:00 am Central on Classical MPR.