Always will be Sargeras. Apparently a majority of the cosmos was just eradicated by him and his Legion.
He was always envisioned as the big bad, the boss of all bosses.
The Trigon, The Thanos, The Darkseid, The Galactus, etc the final boss. Though didn’t work out that way in practice sadly.
I think for me that would probably be the worst villain. I recall Deathwing being hyped up to be this big spooky threat that’s going to ruin our lives and make the world Hell on Earth…
Honestly Cho’Gall was more intimidating with his cartoony yet manic behavior.
We are the player. we are mass murders who would kill a school house full of children for a few coppers.
OH it’s bugged lol!! Like some people’s achievements saying they earned it in 1969 
Worst? Sylvanas. She’s a terrible villain.
I feel sad for Sylvanas’ character development.
It seems that she was set to be this awesome edgy anti hero, but then the Cataclysm leveling revamp happened and since then she started to be turned in to this mustache twirling villain ever since.