Darcy and Elizabeth's marriage could be considered "exemplary" because, in comparison with the other relationships in Jane Austen's novelPride and Prejudice,their financial, sentimental, social, and family situations are more solid and secured than those of others.
Chapter LXI explains how, once Lizzie is established in Pemberley, things could not be better. When you think about it, it is nearly a dream come true: to leave behind the meddling Mrs. Bennet, enter a semi-palace like Pemberley, and earn the love of Darcy's sister, Georgiana, is more than anyone could ever wish to receive in a new relationship.
Moreover, Elizabeth keeps to stay the way she is: she does not become submissive and introverted (as we could suspect that Jane would), but instead she is now even more free to exercises her freedoms; those very same freedoms that at first seem to limit her quest for the love that she really wants to get.
Pemberley was now Georgiana's home; and the attachment of the sisters was exactly what Darcy had hoped to see. [...]; though at first she often listened with an astonishment bordering on alarm at her lively, sportive manner of talking to her brother. [...] By Elizabeth's instructions, she began to comprehend that a woman may take liberties with her husband, which a brother will not always allow in a sister more than ten years younger than himself.
So, from this quote we learn that Elizabeth takes liberties with Darcy, and that such dynamics actually help bring them even closer together as a couple. This certainly lets us see that Elizabeth is a happy woman, in a very well-to-do environment and surrounded by people who love her.
In contrast, Lydia and Wickham continue with their dysfunctional relationship. After all, what starts in chaos, ends in chaos. Hence, they live above their means, request help from Lizzie and Jane, annoy them at times, and it is clear that Wickham has ceased to love Lydia. Lydia, oblivious to all, continues with life as usual.
Their manner of living, even when the restoration of peace dismissed them to a home, was unsettled in the extreme. They were always moving from place to place in quest of a cheap situation, and always spending more than they ought. His affection for her soon sank into indifference; hers lasted a little longer; and, in spite of her youth and her manners, she retained all the claims to reputation which her marriage had given her.
Finally, you have Jane and Bingley who are as happy as Lizzie and Darcy.
Mr. Bingley and Jane remained at Netherfield only at Merryton twelvemonth. So near a vicinity to her mother and the relations was not desirable even to his easy temper, or her affectionate heart.
However, we know how long it took them to finally come to terms that they were interested in each other, in the first place. We also know that Bingley is so easy going, and easily led, that he even lets Darcy move him away from Jane back in the London days. So, we never really know how "Jane's affections" or "Bingley's temper" will affect their marriage when they speak so little.
Hence, out of all, definitely Lizzy and Darcy could be considered exemplary.