In no particular order.
Ner’zhul (MU): I like how he went from Kil’jaedens punching bag to actually being a major threat to the demon lord and in doing so, became one of the few characters to actually deceive The Deceiver (another other being Sargeras). How he could read Arthas like a book and knew which buttons to press via Mal’ganis. How he used Archimondes temperament to deny the other Dreadlords to launch an investigation following Mal’ganis’ untimely “accident” or “it was necessary to fully corrupt Arthas”. Depending on how you want to read it. Ultimately Ner’zhul was vital to the Legions defeat at Hyjal, even if our heroes (Thrall, Jaina, Malfurion, Tyrande and others) were not aware of it. I am a sucker for villain vs villain stories.
Garrosh: Mainly due to how his encounter draws fully from his negative character arc. How he started off consumed by his fears of repeating the same mistakes as his father, thus doubting his own leadership abilities and in doing so, fell to despair. Only for Thrall to renew his sense of Pride but that pride blinded him to the path that would fulfill the fears he had. He gave himself willingly to a “force beyond reckoning”. Threw away the weapon that had belonged to his family for generations and became synonymous with the name “Hellscream” for an Old God copy. His encounter calls back to his fears, doubts and despairs with the intermission phases during Phase 2. And when he is in a corner, he double downs and fully embraces Y’shaarj. After the encounter he shows some sense of regret but that is flushed away when given the chance to create a new horde, free from the Legions stain before it could even be made. Also he was WoW’s first expansion main antagonist that did not come from the RTS games.
Khadgar: I mean what is there to say? He is Dadgar. His young adulthood was taken from him by Sargeras / Medivh. So it is only natural that he acts a bit of a child / teenager but when things get serious, he drops the act and becomes the adult in the room.
Sire Denathrius: In a world where every villain must have a tragic backstory or they have a point but went too far in order to be considered “good”, it was refreshing to have a charismatic pure evil villain. I also love how is only real weakness is his genuine love for Renethal. I will always see him sending his ‘son’ to the Maw as more of a fatherly punishment i.e. you have to spend time with that one family member no-one really likes. I mean after all, Mal’ganis does say that no harm must come to the traitor Prince because “it would upset the master”. Easily the best thing to come out of Shadowlands by a large margin.
Queen Azshara: Prideful, Wrathful and ever Radiant. Everything an Evil Queen should be. I loved her characterization in the WotA novels and I was happy BFA kept that. I wanted more Azshara in BFA so hopefully Midnight, the expansion she is speculated to make her grand comeback is just as delightful from what little we saw in BFA. Much like Garrosh, I think her encounter is one of the best Blizzard has done for mixing story telling into the mechanics of the fight. The fact that it starts off with a bit of theater, transitioning into her getting bored so she throws herself into the ring. After having her fun, she brings out her gambit only for two upstart mages to even the playing field. Now she has to fight for her life for probably the first time ever. She fails to kill her enemies but does succeed in freeing N’zoth. However this was not the outcome she was hoping for. Her warbringers video is easily my favourite for those kinds of videos. The back and forth between her and N’zoth is always fun to watch. And how in what might have been her final moments, her pride quickly turned into panic and fear. Fear that she might have stepped over a line and bit the hand that could’ve fed her, saved her.
AU Gul’dan: Troy Baker at his best. Nuff said. But look at Daddy D’s reasons if you want more. Well, mainly the “fun evil villain” reason.
AU Blackhand: Oh boy that Talador cinematic and what a boss encounter. Easily the best from WoD’s for final raid boss.
Neltharion / Deathwing: Will of Iron and Stone, reduced to molten flame by the will of an Old God. Cursed to feel pain whenever the planet cracked. I love how Blizzard shaped him from a generic evil dragon with no substance into their version of Atlas. Burdened to hold the world on his shoulders. Only to be crushed by the weight. Driven to do evil acts on the false hope of being cured of this “curse”. Only in one possible future, to be cast aside as a disposable pawn. How many lives were ruined because of this?
King Varian Wrynn: “War. It’s all you’ve ever known isn’t it?. You were so young when you survived the First War. No wonder you’re so equipped in spirit to fight as well as you do. War is in your blood… You were forged by it!” - a slightly changed quote from a character from Star Wars. Bonus points if you know who says it, the show it is from, the episode name and who the original quote was about. But the quote does work for Varian. He was forged by war. Surviving the First War, becoming a refugee during the Second. Hoping to have a peaceful life when Stormwind was reconstructed only to be kidnapped when journeying to a peace summit in Theramore. Ripped in two, with one half becoming a gladiator slave while the other was sold back to Stormwind to be used as a puppet by Onyxia. And when he was made whole again, the scourge came a knocking. Losing his best friend and trusted guardian of his son to a betrayal. Then once again thrusted into war against the Horde. In his final moments he made the ultimate act of defiance against the Legion. Died fighting for his people, his son and his world against evil itself.
Onyxia: Easily Blizzard best written villain when it comes to a schemer and manipulator. She learned from her father after all but almost succeeded where he had failed. Wasting Stormwinds money on pointless military campaigns to expand a border that was already far extended due to Stormwinds defeat in the First War. In doing so, denied the Stonemasons the gold they deserved for their hard word restoring the city. Played the House of Nobles and the Defias Brotherhood as if they were mere puppets on strings. Sparking a civil war and working alongside her brother to launch the Blackrock orcs on an attack path into Redridge. While also sending other soldiers to the frontiers against other threats and even the Horde in various theaters of war. Leaving the defence of Stormwind and its borders to ununified militia groups. All dealing with their own problems to keep them from figuring out the enemy from within.
I might add more in the future.