Different for everyone. Common picks are "Imprint", "The Fair-Haired Child", "Jenifer", "Cigarette Burns", "Homecoming" and "Family".
"Imprint" gets a nod just because Takashi Miike went so far into the extreme that they couldn't even air the episode!
"Incident On and Off a Mountain Road" Ellen taking the fight to Moonface and Buddy. She manages to kill Moonface in a strait up fight and shoots Buddy, but not before giving him his masters "Shhh". The best part is that she turned the tables on them when Buddy lured her into a trap.
"The Fair-Haired Child" When Johnny is fully resurrected, it turns out he made the same Deal with the Devil his parents made, this time to resurrect Tara, and had a talent in bargaining. You see he managed to narrow the deal down to two adults rather then twelve kids. Seeing Johnny's parents Karmic Death was pretty satisfying, and Johnny and Tara got a happy ending.
"Homecoming" The ending with the zombified Murch announcing he and the other zombies will show anyone who sends their brothers and sisters to die for a lie, will know the true face of Hell.
"Family" When it's revealed Celia and David are parents of one of Harold's victims, and have been luring him into a trap all along.