Ben 23: Omniforce/YMMV

Catharsis Factor: Out of all the Villain Deaths, the most satisfying may be the death of the scumbag known as Nightmare, as seeing him beg Ben for his life while Ben is beating his face in feels so good. Complete Monster: See here: https://ben10fanfiction.wikia.com/wiki/Monster_/_Ben_23%3A_Omniforce
 * Tragically subverted with Gideon killing T'ket, as not only did she set off bombs in all of her soldiers (and Gideon's brothers) to painfully disintegrate, but right before she is vaporized she is laughing at the pain Gideon is in. Ben 23 beating her up and defeating her, however, plays this straight, as the souls of everyone T'ket has either directly or indirectly killed as well as the power of all remaining heroes (including all the Iconian Heralds, who have defected after realizing that T'ket is irredeemable, as well as the surviving Voth) help Ben 23 finally end the nightmare that is T'ket forever! 

The Woobie: OH MY GOD. There are to many to count, both from the show and the prequel.