People like to condemn the lore of the later expansions while looking back on the past with rose-tinted glasses, but I would personally argue that every expansion has had something going for it in terms of lore. Asinine gameplay systems and class changes aside, the lore of the game is the one uniting force that has kept me invested throughout the many years I've played this game.
That said, nothing's stopping us from playing favorites. If you were to ignore everything else about an expansion and judge it based solely on the lore (story, setting, etc.), which expansions would you rank at the top?
My favorites would probably be Legion and Wrath. The Nordic themes of Wrath were enough to make me fall in love with it, but it also had one of the coolest villains the game has ever seen. Legion, on the other hand, was the first time when the game felt like it was actually making leaps and bounds to move the main story forward, and there was a wide array of smaller storylines, all of which introduced interesting characters and stories. I was especially a big fan of the night elf lore (Azsuna, Suramar, and Black Rook Hold).