That's a tough call, Don B, and I'm sure IGI and Ruth have some favourites they will recommend
But setting my own criterion of "operas I would buy on cd in preference to TSAR SALTAN" (which I don't dislike, but it's something of an overgrown byway of the Russian repertoire)...
Tchaikovsky:# THE ENCHANTRESS ("Charodeika") - some lovely music, and a great poisoning scene, redeems a rather potty plot about a widow who is loved by both a father and his son.
# THE OPRICHNIK - I know IGI isn't over-fond of this one (and the composer himself grew to dislike it too). However, it's a glorious romantic wallow in desperately dastardly deeds.. a poor but honest young man needs to advance himself in order to win his sweetheart's hand, so he joins the infamously wicked Secret Service (the Oprichnina) of the Tsar. His would-be mother-in-law is bitterly opposed to this, but the story plays-out against a family grudge against the boy. As you might expect, a Jacobean-tragedy-style stageful of stiffs is the inevitable conclusion. Stupendous music especially in Act One, with an outrageously over-the-top solo scene for the mother-in-law (hefty Russian mezzo), a real scene-stealer if you have the right singer to hand. The entire opera is very neglected in Russia.
# THE MAID OF ORLEANS ("Orleanskaya Deva") The huge success of this opera in France accounts for it often being performed in French, especially Joan-of-Arc's lament "Adieu, forets!", which is the "hit number". Another one hardly performed in Russia either.
# CHEREVICHKI aka THE SLIPPERS. Amazingly, a comedy by Tchaikovsky? Another retelling of the popular Russian fable Vakula The Smith.
Mussorgsky:# BORIS GODUNOV and also KHOVANSCHINA - both are repertoire staples. Mussorgsky left both in semicomplete states which (questionably?) needed editing and revision after his death. The David Lloyd-Jones "authentic" version of BORIS was popular for a while, but I have a sneaking fondness of Rimsky-Korsakov's souped-up version, since it includes the "Polish Act" which is missing from the "original" version. This substantially alters the plot - instead of Russia falling into anarchy (as in the original) it becomes a fight for the throne between Dmitry the Pretender and Marina Mnishnek, the avaricious Lithuanian Princess... and a battle for the souls of the Russian people, since Marina's "backer" is the Roman-Catholic Cardinal, Rangoni.
Rimsky-KorsakovTHE SNOWMAIDEN ("Snegurochka" - she is actually "Little Snow Maiden" in the Russian title). Rimsky's fairytale fable is an allegory of lost innocence and a parable on the corrupting ways of modern society... those would marry her for her beauty simple cause her to melt.
and here is the ex-Mrs Torheit singin' it... she melted-away when I married her too, ho-hum...THE TSAR'S BRIDE ("Tsarskaya Nevesta"). This opera is ONEGIN's closest box-office rival in Russia, where it is immensely popular. Personally I'm unimpressed by this cardboard tale of backstabbing and poisoning in Ye Olden Dayes, which makes YEOMEN OF THE GUARD look like a real masterpiece by comparison.
SADKO - another of Rimsky's fairytale pieces of Russian lore, written in the post-assasination ere to spur national pride. Ther are eight or nine more by Rimsky, but they are rarely done... I particularly like MAY NIGHT ("Maiskaya Noch'"), which has a happy ending and deals with some Rusalka-like material with waternymph heroines of once-crossed lovers etc. At least it isn't chainmail knights as usual in Rimsky
THE LEGEND OF THE INVISIBLE CITY OF KITEZH & THE MAIDEN FEVRONIYA - this one
is chainmail knights as usual in Rimsky
RachmaninovALEKO - people forget Rachmaninov wrote opera, but he only ever had the chance to do this one... thereafter happenstance of life prevented him from following-up his graduation-project success. A strange story of Aleko, who is jilted by a gypsy girl in favour of another... he plot to murder them both in revenge.
RubinsteinTHE DEMON - This is one of those "the gods dispute about the feeble efforts of man to live a good life" plots. The Demon wagers with the Angel that there is no such thing as redemption through love. In the course of his bet he falls in love with the subject of their wager himself, an innocent girl called Tamara. The title role was written for the star young bass of the Mamontov Private Opera (discussed over on TOP), Fyodor Chaliapin... his lullaby "Ne plach, ne plach, ditya" ("Don't cry, don't cry, my child") was an enormous hit at the time, although forgotten now.
DargomyzhskyRUSALKA - although Dargomyzhsky is most well-known for THE STONE GUEST, I think his RUSALKA is a better opera entirely. Some nice music in this indeed. Also, if you can find it, there are his sketches for an incomplete operatic version of MAZEPPA, some 20 years before Tchaikovsky set the piece.
