Thunder King holds a special, lightning-charged spot in my heart.
Garrosh just because of the long and winding path of cause and effect he took us through, from TBC to WOTLK all the way to Pandaria.
Gul'dan for being an extension of that and the character development "AU Gul'dan" got to set him apart from "MU Gul'dan"
She certainly took me off guard, and at first I thought "eh just another SURPRISE! heel turn".
But then I played an alt through and I realized just how cleverly written she was.