Rate from best to worst:
(I picked them not based on final bosses but main antagonist in each expansion)
Classic- C’thun (current retail) Kel’thuzad (past retail)
BC- Kil’Jaeden (Illidan now reformed)
Wotlk- Lich King
Cata- Deathwing
MoP- Garrosh Hellscream
WoD- Gul’dan
Legion- Sargeras
BFA- N’zoth
SL- Zovaal
My list;
Why C’thun and not Kel`Thuzad?
You went with the very last boss of every era except classic.
I went for the retail version of villains. 
He was the last boss in retail. 
Best: Lich King and Deathwing. They harassed you the entire expac and it was very satisfying to kill them at the end.
Worst: the rest
Because Kel is secretly the good guy for the franchise. He tried to kill us all before we could see how bad it would get.
Lich King
Illidan
Deathwing
Garrosh
Sargaras
Kil’jaeden
N’Zoth
Gamon
Hogger
Steve Danuser
…
…
…
…
…
…
…
…
…
The Jailer
Good list. 
My only complaint is Hogger is too low, he is clearly SSS Tier.
tbh they brought back kel like 3 times in the game already, that’s why I didn’t mention his name.
I know, he keeps trying to save us from this horror.
1: Garrosh Hellscream
2: Lich King
3: Illidan
4: Kelthuzad
5: Sargeras
6: Archimonde
7: Deathwing
8: Nzoth
9: Zovaal
I really hope it turns out Denathrius is the 5D chess playing mastermind behind SL’s plotline, just so I can say that, despite all the criticism SL got, it hilariously turned out to have the greatest villain out of all the expansions.
Because I unironically think Denathrius is the best villain WoW has ever had.
True, maybe Denathrius is going to come back for a future expansion? I dunno, but since 9.2 is the final major patch of SL, it’s safe to assume that zovaal aka robot man is the main villain of this expansion.
Yeah but in my eyes final boss =/= main villain. If that were true, the Lich King would technically not even be the main villain of his own expansion. Archimonde in WoD had only a minute of screen time in WoD outside of his boss fight. It’s pretty safe to say both Grommash and Gul’dan were more deserving of the “main villain” title. Heck, you even listed Sargeras as the villain of Legion (which is true) despite him never being a boss fight. So I’m not entirely sure why you gave that title to Archimonde for WoD…
Bottom line; if next expansion (or one even later) reveals that the Jailer was just a puppet doing everything Denathrius told him to do, I’d feel pretty comfy retroactively considering Denathrius to be the main villain of SL.
I was gonna say, I think Gul’Dan is the one that pissed me off most.
I updated the list now, your suggestion made a lot of sense to me since Archimonde wasn’t even given that much screen time.
All of them have been pretty mediocre. TBC Illidan was a huge disservice to the WC3 character and WotLK Arthas was a cartoon version of himself.
Zovaal is by far the worst. What a complete disaster, others are rightfully eviscerating him better than I could.
I much preferred Onyxia as a main villain in Vanilla.
I liked the whole subterfuge angle of her story in Stormwind.