Classic has fun and many interesting and even hidden quests and quest lines.
What are some of yours? I’ve been on a quest to complete them all so been having a blast.
Some of my favorites:
Dig rat stew - totally hidden fun quest
Battle of hillsbrad - fun line
Sacred flame - fun line
Samoflange - neat and rarely done
Battle of darrowshire - coolest scene at the end
The one where you try to rig a race in shimmering flats through blackmail, extortion, and drug running warcraft cocåine (dream dust) for some off shore banking details
Alliance Onyxia Attunement.
I really like the Riddle Quest.
I have helped multiple Alliance players (because it’s Alliance exclusive) through the Faerie Dragon Pet Questline. It starts in Feralas and ends in Hinterlands. Best Pet, in Classic, IMHO #FaerieDragonClan 
Gann’s Revenge because he teaches you how to make gelignite to blow infidel Dwarves to the Makers. Definitely a blast from the past to see a quest using actual chemistry compared to just in-world creations.
I always liked Mankrik’s little story.
Up until him all your quests are like, kill just a couple of these things, or a handful of those guys…like you’re killing things, but only a small amount. And then you meet Mankrik, find out his wife was beaten, probably defiled, and murdered and his dialog is deadly serious about wanting you to kill basically all of them. Like, stone cold, murder them and bring him 60! Trophies from their corpses. Even if you get the quest at like 18, it’s such a dramatic shift in tone and numbers of things to be murdered that it just feels so much different from the other quests around it.
Raene’s Cleansing. Gota go furblog
That’s easy: Tirian Fordring’s quest line.
just all of Westfall and Duskwood.
Most of Duskwood. Stalvan and Embalmer(Stitches) especially
Redpath
Linken
Linked quest line is pretty neat yeah.
Not necessarily a quest line but more of the story of a few zones.
The Elwynn > Westfall > Redridge > Duskwood story is amazing. As a life-long horde player who didn’t level any Alliance characters past 10 pre-wrath, and who finally had the chance to do this during classic, the story is incredibly immersive and you feel like a human in a fantasy world.
To answer in the spirit of your question though, Battle for Hillsbrad was neat.
Oh yeah I love that too. I frequently make new toons on different servers for fun start, different factions too. That lore in elwyn forest and west fall is an amazing journey. Love how it leads into deadmines and into storm wind to spy on some dude.
A new plague!
A little too ironic in today’s world.
I think the horde test of faith quest lines where you go through a series of trials that starts with leaping off a bluff in thousand needles is pretty cool
The chain of quests in the Blasted Lands starting with the fallen hero of the Horde and ending with confronting the dreadlord of the zone was pretty cool.
I got to do that one.
Gurubashi arena quest. Cause chances are you got to kill your own faction to do it.
Good question.
I may have to go with the Scarlet Oracle questline for Horde from Nathanos Blightcaller.
You get apples that can turn you permanently into scarlet crusade members.
If you don’t finish the quest, you can keep getting more apples which is fun.
Similar to the horde ony chain (which is absolutely more involved and difficult) to the gnome witch illusionist turning you into a scarlet member. Except you don’t have to keep going back when you inevitably transform back when you do anything.