If i wanted to be edgy id say
“You are all, Nothing!”
i hear it when i win a battle against some easy NPCs.
Thick44 influence i suppose.
many times on the forums when someone takes a strong stance on a subject i hear the forsaken line:
“…and you are?”
also for no apprent reason other than the weird accent
“Every zing must BERN!”
Every time Blizzard added a grind or feature I felt was a waste of time I heard Ebonhorn saying “No, I won’t be doing that.”
“Trash! Trash! Trash! Trash! Trash!”
Tempest Keep was merely a setback!
The amount of times we had to run that dungeon trying to get one of our tanks the Mark of Dargrul resulted in that line being a fairly common response to things that we would not be doing. It’s a great a line.
" You are now the Slayer of stupid incompetent minions." Or something to that effect.
Grand Apothecary Putress at the Wrathgate.
“Did you think we had forgotten? Did you think we had forgiven?” Gives me the chills to this day homie.
I always thought the roleplay with raest and karam magespear for the mage tower appearence was pretty epic. One of the few things I liked about legion.
‘‘Augh… Their filthiness stains me!’’
Budd: I’M SAAAAAAILING! Ooh a shiny…
Budd nooo!
“INCINERATE THEM, MINIONNNNNNS!” lives rent-free in my head since Cataclysm 
“Enough! Your gyrations sicken me!!”
Lol. This boss fight was when I turned dialogue on for the first time. I laughed almost every time. I play with my kids and one of them used to randomly yell this quote at me.
I also laughed almost every time during that part of the dialogue. Dude missed out on the fun but I respect his answer lol.
I am my scars.
“It’s one banana Michael, how much can it cost, 10 dollars?”
I know not WoW related, but Jessica Walter will forever live in my heart.
This one has so many uses
Everything burns… including you…
That fight is so damn long, that voice line is so damn repetitive, and he is so damn monotone, that it drills into my head and makes me want to scream
“You are not prepared!”
also
“Squawk! Pretty trinkets!”
“You think you’re better than me?! Alright, let’s go tinker boy!” -Gallywix