1. Mists of Pandaria - unique setting, a compelling plot that didn't revolve around saving the world and a focus on world building. Each zone was legitmately unique and memorable. I genuinely cared for the people of this new world who were caught in the middle of a grey-and-grey morality war between outside forces. The art style is fantastic, the quests are creative, and all around made for the best expansion I played. The music is the most memorable in the game.
2. Wrath - a cohesive world and a solid, epic storyline. General gameplay was fun, but not inventive like MoP. The music is the most memorable behind MoP.
3. Warlords of Draenor - as much hate as it gets, the expansion was overall more enjoyable than BC and and Cata. Polish is amazing and the cohesiveness of the main plot is astounding for an MMORPG. That said, only three zones - Frostfire Ridge, Shadowmoon Valley, and Spires of Arak - were memorable, with Gorgrond being a bland smattering of green and orange on bumpy land, Talador being a generic plain with large trees spread throughout, and Nagrand just being a plain. Tannan was the worst fore me, as the fel-green energy was indistinguishable from the already acrid-green skybox and dark green jungle. Unlike MoP, which took liberties with it's non-war focused quest designed (due to the fact that half of MoP was helping out Pandas with their everday problems as opposed to constantly saving the world from dragons), WoD's gameplay was most of the same "kill-orcs-to-win", and boring. Garrison was a great attempt at introducing a highly requested (player housing) feature, even if the long term execution was botched.
4. Cataclysm - more than anything, it was the old world changes that made this expansion for me, as it not only made the old world playable, but also effectively gave us 52 zones of new content to play through, even if the vast majority of it was bland and uninteresting. I will never forget the Dragonlance-esque plot in Azshara where I fought off a dragon invasion with Kaelecgos at level 20! (That was one of my first ever WoW experiences and was THE MOMENT that got me invested in WoW's lore and story).
5. Burning Crusade - I never played WoW when this expansion was active, and having played through Outland years after it's release, I find it heavily dated compared to later expansions, and generally uninteresting. So much so, that I've found the new starting zones to have more compelling content than the actual endgame of BC.
I have not played Legion yet, though given that theme is more of the same from Warlords, I'd imagine I'd place it around #3.