Probably the sweetest revenge out there. Arya Stark killing Walder Frey and revenging her family, Robb Stark, Catelyn Tully Stark, Talisa Maegyr Stark and the Blackfish.
Not only did she kill him, she killed his 2 heir and children and carved them and made them a pie, which she offered him. It seems all that blindness and training in the house of black and white with Jaqen H’ghar has finally paid off.