If you mean which race has gotten the most lore or best treatment then it's Orcs by far. This game should be renamed World of OrcCraft.
In terms of which one is my favorite or what I think is the best lore, I would say Belf's. I have always been partial to the Blood Elves since the days of WC3. Also I'm a fan of the Highborne and non-WoW Troll lore. The Orc and Draenei lore is pretty good if you ignore Thrall 'Jesus' and everything he did post WC3. The Titans/Dragons and Sageras are also interesting, one of the reasons Uldar is my favorite raid. Only lore I really never cared for is Gnome, Night Elf, Human and Tauren. Other than that most of the races have something interesting about their lore.