I think I’d probably pick the old classic which is Illidan’s:
“You are not prepared!”
-I don’t know, it always just had its own touch and epicness.
Also I really liked what Gul’dan had to say before we got to face him. The way he spoke made me want to finish him off even more.
Men, women, and children… None were spared the master’s wrath. Your death will be no different.
I just find Falric’s voice there brilliant.
All Kael’Thas monologues
“Money orders require verification by signature, photo and…BLOOD!”
All the VA in Taz is great but this guy is wildly entertaining.
Can you feel the cold hand of death upon your heart?
-Sindragosa
edit: As for the why part, I really like her VA.
Did someone say Kael’thas monologue?
Finally, a captive audience!
-Bronjahm, FoS
You are weak, powerless to resist my will!
-Lady Deathwhisper, ICC
The ground runs red with your blood!
-Deathbringer Saurfang, ICC
Hehe 
Oh ! I forgot Sindragosa !
Weird fact, it’s a homage to Jim Morrison of ‘The Doors’ … the quote he made is “I am the Lizard King! I can do anything!”
My personal favourite boss quote, I mean there are a lot of good, and intimidating ones, but my favourite has to be because the reverb effect makes it sound weird…
Lord Jaraxxus…
Now what he actually is meaning to say is “I will rend you, flesh from bone”
Sounds scary, right?
Next time you listen to it (If you rerun old content) what it -Sounds- like he is saying is":
“I will end you, Flesh Trombone”
It’s like…“What did he just call me? a Flesh Trombone?” That’s possibly the rudest thing without being actually rude… What even is a flesh trombo-oh my gods, he’s attacking me, I can ponder that later!"
I promise you, when you hear that dialogue as ‘Flesh Trombone’ you will never be able to unhear it…
“Prepare yourselves, the bells have tolled! Shelter your weak, your young and your old! Each of you shall pay the final sum! CRY for mercy! THE RECKONING HAS COME!”
Kelthuzad make patchwerk his avatar of war !
I am the lucid dream.
The monster in your nightmares.
The fiend of a thousand faces.
Cower before my true form.
Goosebumps every single time!
One of many favorite voice lines i have is Mogul Razdunks intro line from The Motherlode.
‘‘Do you have any idea how much property damage you’ve done?!’’
Fire! Fire! … Aim, then fire!
‘‘I am the lucid dream…the monster in your nightmares…the fiend of a thousand faces! Cower before my true form! BOW DOWN BEFORE THE GOD OF DEATH!’’
“Da shield be down! Rise up, Atal’ai! Rise up!”
Uh, because nostalgia.
This may be cheating, as this is from a trailer, but…
"This is OUR land: you wanna stay, you stay here forever!
We gonna bury you here…"
Chills!
“Let the games, begin”
Good old Neffy never disappoints.
Also Prince Malchezar in kz:
“You face not Malchezar alone, but the legions I command!”