I'll seem like a simpleton when I admit this, but that's okay.
I love Ragefire Chasm.
It's so simple, easy, and when you're a low level you meet new friends through these dungeons so it's a pretty nice networking dungeon off the beginning.
Next in line would have to be Shadowfang Keep because of all of the interesting bosses and the size of the dungeon is a pretty good run.
Maraudon for me, I love its varying aesthetics, and that it's segmented into all these different parts which can be accessed through each other. Usually Maraudon runs can also end up being very long when no one has the scepter, so lots of great memories have been made in that dungeon.
Sunken Temple would be my runner up.
BRD obviously, as many also chose. Needs no explanation.
Also really like Zul'Farrak. The theme and bosses were really fun. Have good memories of that place.
And Wailing Caverns is just a classic. I love how a group of 'noobs' with few skills run around that snake pit, collecting their first greens and occasional blue. Everything that drops there feels like such a big upgrade.
EASY! Scarlet Monastery, for sure. I don't think I've ever felt more at home in a dungeon. I tend to main priest and the entire inquisition/catholic church aesthetic fits perfectly with my priestly fantasy. I remember spending hours trying to get that bloody tabard of the Scarlet Crusade. In addition, that room with the secret sliding door in the Crusader's Chapel blew my mind when I saw it for the very first time. Can't wait to get in there and experience it all over again. IMO, Whitemane's Chapeau is the definition of vanilla priestly aesthetic.
Scarlet Monastery - it's got something for everybody, and prior to level 60 it's the only dungeon that I've always ran multiple times on virtually every character I've rolled.
Deadmines. An absolute classic and hands down probably the most iconic dungeon of World of Warcraft. Everyone knows it! It was so neat crawling through this claustrophobic series of caverns and tunnels, fighting goblins, miners and mages to finally blast open this door to a huge pirate ship inside a massive open area filled with pirates and their parrots. Truely surreal and I don't think any other MMO has quite captured a similar experience and community aspect as that dungeon has.
I'll narrow it down to my top three:
1) Stratholme - the environment in the burning city was just incredible. From the secret bosses, to the two sides to run,
to the raid portal to Naxx that never came to fruition, to the lore from WC3... this is my favorite dungeon.
2) Scholomance - another excellent environment and backstory nestled into a very challenging dungeon.
3) Black Rock Spire - this is a combined upper/lower dungeon. Crossing the last bridge to Drakkisath, looking down and seeing so many
different floors of Black Rock Spire has always impressed me.
Check it out: https://www.scrollsoflore.com/gallery/index.php?cat=7
The gallery has like every single piece of official concept art and renders etc. for wow classic.
I loved Scholo. Ran it countless times with my fury warrior friend - Belusterk. It was just a well put together environment and I dug it.