You know. I don’t know. I’ve been playing and, generally, enjoying this game for 14 years but even still I cannot answer that question - what are the greatest hits of WoW content? You kinda had to be there at the time the content was relevant with 4/9/19/39 other people to really get the experience.
I have fond memories of Pandaria, but they ruined the Vale of Eternal Blossoms. And you’d have to do everything the expansion offers, in order, to begin to understand the entire story. Which, again, might not really be effective without 4/19 other people, anyway.
I thought Legion was probably the best expansion. But you’d have to do it all, including the full Suramar campaign and the various raids. The Deathknight order hall campaign was probably the best one. The quest where you get to play as Illidan in the Black Temple is certainly fantastic. But it’s awesome because it’s making fun of the player base. In fact, any quest with Johnny Awesome is awesome. Here’s there during the Illidan raid. Well, he’s afk, of course. Because he’s Awesome.
He’s also there during the Postmaster questline, which was just fantastic, maybe the best quest in the game. Yeah, any quest involving Johnny Awesome is probably pretty awesome (be sure to watch the first video), though I regret I don’t get to see him very often as he’s a belf so he’s horde.
The Day Deathwing Came is pretty good.
You should also do Icecrown Citidel, kill Arthas, and watch the cutscene. Do it before the Postmaster questline. And the Deathknight order hall campaign.