I would choose the marriage of Levin and Kitty in Anna Karenina.
In a novel which, like others of Tolstoy, critiques the institution of marriage and shows the unhappiest examples of it—not only that of Anna and Karenin, but that of Dolly and Oblonsky as well—the union of Kitty and Levin is a bright spot. To summarize the important relationships in the story, I'll provide the following list of characters relevant to this question (using only first or last names, since the full Russian names can be difficult to keep straight for most of us who are English speakers):
- Karenin, Anna's husband
- Kitty, Levin's wife
- Dolly, Kitty's sister
- Oblonsky, Anna's brother, married to Dolly.
- Vronsky, Anna's lover
In the beginning of the story, Levin, a young landowner, is in love with Kitty, the daughter of Prince Shcherbatsky. Kitty, however, is in love with Vronsky, a military officer, and expects an imminent marriage proposal from him. When Levin proposes marriage to Kitty, she politely but somewhat coldly turns him down. Vronsky, however, has no interest in marrying Kitty or anyone else, but he pursues Anna, despite (or perhaps because of) Anna's being married to Karenin.
Kitty is so distraught over Vronsky's non-interest in her that she becomes physically ill. In the meantime, Levin returns to his estate to nurse his wounds. But some months later, he and Kitty are reunited at a party, and the two find they are almost telepathically connected. When Levin writes out a message to her using only the initial letters of each word, she understands it, and then he understands a similar message written out by her. The two get married shortly after this, and despite the problems that occur in any relationship, the marriage seems to be "happy."
This contrasts sharply with the marriage of Kitty's sister Dolly with the philandering Oblonsky and with that of Anna (Oblonsky's sister) and Karenin. Anna, at the start, has what seems a stable marriage with Karenin. But when Vronsky begins to pursue her, she comes to see that Karenin is a stuffy and unfeeling man, overly precise, sarcastic, and clueless. Even when he sees Vronsky and Anna paying inordinate attention to each other at a party and making it obvious that an affair is either happening between the two or is about to happen (as is the case), Karenin only realizes something is wrong when he notices that other people see that Anna's behavior is "inappropriate." Anna and Vronsky begin their affair, and Karenin takes Anna's and his son Seriozha away from her and refuses to give her a divorce. After several years of living with Vronsky, Anna realizes his passion for her has cooled and that (although he doesn't say so) she's become a "burden" he would probably wish to get rid of. Ostracized by society for having an affair (though no such ostracism has happened to Vronsky or any other male character for the same thing) and thinking herself no longer loved by Vronsky, Anna commits suicide by throwing herself under the wheels of a train.
I mention these details to illustrate the contrast between this tragedy and the marriage of Levin and Kitty. But, as stated above, their marriage is hardly perfect. Levin is irritable and irascible, though a basically good person. Kitty is overly sensitive, especially when, late in the story, Levin meets Anna (who had "taken" Vronsky from her) and seems attracted to her. But Tolstoy's point appears to be that a marriage lasts when a couple does not expect perfection but realizes that a marriage or any relationship is supposed to have problems.