There are a lot of good ones to choose from. I’d probably list it as my favorite per expansion.
Classic: Scarlet Monastery…at least all of it except GY. I like the whole bastion of paladins in the middle of a dying land type vibe I get from it. It also explores the Scarlet Crusade, which has always been a cool faction to me. Plus the final Cathedral set piece is beautiful in its own low res way.
BC: Magisters’ Terrace. I love the blood elf aesthetic and this place is the peak of that. BC didn’t really have a clear central story, so as a blood elf paladin player at the time, the whole story of the blood elves being betrayed by their prince, forgiven by the naruu, accepting the Light, and then finally confronting their price a second time at their ancestral home really resonated with me and made me a belf stan for the longest time. This dungeon was not only fun and interesting (being a mini-TK in a couple ways), but was the perfect piece to end my player story in BC leading up to the Sunwell.
WotLK: Forge of Souls. I love the mini-storyline we got through the three dungeons. I wish we would get more of that. It was a tough choice between this and Halls of Reflection as the escape scene from the Lich King is unique, fun, and thrilling, but Forge of Souls gets my vote as it was the first time many saw the inside of the Lich King’s fortress and it did a really good job of providing us a sinister and foreboding start through our journey into the citadel.
Cata: Deadmines. I didn’t really like the cata dungeons, but the redesigns of the classic dungeons were neat. I think some of the cata dungeons had a really cool design to them. Tolvir, Vortex Pinnacle, Stonecore, and so on were a visual feast, and many of the bosses were…interesting, however playing with randoms at the time is such a horrible memory to me and was the cause of me to fall out of love with this game for a long time.
MoP: Uh…Temple of the Jade Serpent. Nice aesthetic which is something most of the MoP dungeons had. I don’t really have any attachments to any of the dungeons as I came back when SoO dropped and all of the dungeons were old content by then. A real shame as they all seemed interesting…except the mantid ones.
WoD: There are a lot of dungeons I love from WoD, but Grimrail Depot is my favorite for the sole reason it takes place on a train. Cool set piece and mechanics that go along with that. This is seriously a hard choice though. I may seem a little crazy for saying this, but many of the dungeons in WoD are some of the best in the entire game.
Legion: A lot of good dungeons to choose from here, but my pick goes to Black Rook Hold. Love the gothic kal’dorei look though admittedly the end of the dungeon is a bit of a sour spot. Overall I would say Legion is right behind WoD in terms of dungeons (in a few cases at any rate), however I might be a little biased in my views as I’m still a little tired of running a select few Legion dungeons.
BFA: Honestly…I don’t really like any of them all too much. Waycrest Manor is my favorite as it continues the Drustvar story (which is the best in the expansion imo) but dungeons in this expansion felt really cramped and the design with M+ in mind is pretty apparent and not in a good way. It seems like they were trying to avoid another Maw of Souls situation, so while we have cool set pieces and fairly interesting bosses, it just feels like they placed far too much trash in each room. I can’t help but feel exhausted after each run, something I haven’t felt since Cata’s dungeons.