Although my first character was a Troll Shaman, the character that really hooked me in the game was a Nelf Druid, followed by a Tauren Druid. It wasn’t really the class mechanics per se, but the backstory of Druids, Nelfs and Taurens, and well as the starting areas.
Since then, I’m still into Druids, and became fascinated with the Drustvar Druidic lore, and my KT Druid has become one of my fave characters.
Original Death Knight in Wrath. The ones where frost could tank, each spec had an aura that buffed the group, if you died you would come back as a ghoul, and corpse explosion (actually doing damage, not a novelty spell).
Wrath Ret Pally or DW armor pen Frost DK from Wrath.
MoP Resto Druid, Symbiosis was a neat ability.
https://wowpedia.fandom.com/wiki/SymbiosisBFA Prot Warrior: Most fun I’ve had with tanking in a very long time. Survivability was pretty nutty and you did more dps than most dps did on aoe fights with TD. Also Mag Orcs in the elite warfront set looked great as a tank.
Combat Rogue
I enjoyed the simple rotation compared to what is now Outlaw.
I hate pistol shot and between the eyes.
Shadow Priest in vanilla…unkillable flag runner in WSG and MC’ing folks off the edge of the world was just so much fun in the BGs. I was one week away from High Warlord on mine before Blizz changed the PVP setup and removed the titles. I still hate them for that.
Cata and TBC warlock.
Wrath priest.
That’s what I mained then, had tons of fun I remember
Brewmaster in WoD.
Wrath Priest was my favorite as well. 
Warlock since 2009. I switched between warlock and warrior over the last 13 years but I always end up coming back.
Something about being a class that accurately represents my manipulative personality is 
to me.
Seriously though, I’ve loved the class since back in the day when you had quest lines for each demon. When MOP was around and the green fire questline came out, I grinded that for weeks until I finally beat it - felt so rewarding/cool, and the green fire cosmetic effect was something I felt amazing about.
We have some of the best tier sets out there, too.
Coming from FFXI and CoH/Villains, way-way back when. And eventually saw a certain Serenity Now video when poking through infant youtube days. Peaked my interest.
Noticed that Warlock had pets and was curious as it was a caster, but was it truly DPS or a Pet class that is pigeonholed into heals… So bought hardcopy of WoW and TBC from KBs and joined up about a month or so right after TBC officially launched.
Been Warlock (all specs) ever since. Have others classes here and there (Resto Shaman,l and SV hunter) but Will aleays be a Warlock. A Hard Lock Life.
Cataclysm Holy paladin. during dragon soul. You did enough dps to duel people using holy shock
I remember just never going oom. EVER. cast judgement get 12% of my mana back over 4 seconds or something like that and judgement has a 3second cd so you never had to use water to get mana back. NGL i really like the theme of shadowlands bastion paladin prot right now though.
Legion Shadow Priest FTW. Surrender to Madness was the greatest thing to ever happen to WoW.
I’d have to say Druid, I always go back to it and I always enjoy playing it.
I can’t make up my mind between MW monk and Guardian Druid.
I love healing more but I love playing bear almost more.
Mage is fun too. Ahhhhh
Let me Begin with since wotlk I have played Hunter, one of best and most stable classes. Alot of nerfs over the years but one of most fun. Druid for tanking hands down spell casters warlock hands down evul lil bastards esp in pvp. warrior too easily rooted and killed so waste pali once sheilds down squishy from pvp side . DK hands down for melee pvp . Rest I dont bother playing too many 3 button mash rotations for my liking.
Vanilla Rogue - Open world pvp was fun and new at the time and you could do some crazy things as a rogue.
Burning Crusade - Warrior tank, just enjoyed the way it played and there was a big difference in skill level between warriors and also tanks in general making you stand out more if you were good
MoP - Prot Paladin while leveling
I’m beginning to like Fire Mage. I finally figured out how they work. However, my favorite remains Beastmaster Hunter.