I've read several 18th/19th century epistolary novels - some pretty bad/tedious and some quite good, but de Laclos' is definitely the masterpiece of the genre. It's extremely refreshing for a novel from the time to be rid completely of a narrator and a grid of interpretation (especially when moral writers of the time tended to be heavy handed in spoon feeding you the way they intended everything to be interpreted. Balzac is often guilty of that, but not only him.). The epistolary device leaves it all in the hands of the reader (though Laclos is devious in a good way), and Laclos was really great at playing with perceptions through the novel. The structure and the layering/polyphony, the whole POV game is really much better done in Les Liaisons Dangereuses than in other novels of the genre, and there's also very good work done on making Valmont and de Merteuil's style and prose unique. I would say it's also the most interesting of the Libertine novels, not just the best epistolary novel.

I've read it a long time ago, but it really left a very good impression on me.