My first one would be Clanden. He is the guy who works with the Omertas. Sick mofo.
I loved killing Clanden so much that I saved my game right before I killed him and reloaded just so I could kill him multiple times. I've beat him to death with my fists, let Arcade beat him death while I watched, shot him in the head, sliced him with a switchblade, shot him and blew apart his body, blew off his head and put it in the toilet... etc. I don't know, man, I just loved killing him.
I do love going on unsaved killing sprees at Gomorrah and The Ultra-Luxe. At the Ultra-Luxe I love killing the Gundarson's. I hate them so much, especially the son. At Gomorrah I love watching Arcade kill Big Sal and Nero with the flamethrower. I love Arcade.
I'm really looking forward to killing Antony at the fort, as well as the rest of the Legion. I've been biding my time.