Nicky.

Nicky, easily. He wanted the same thing Ginger wanted (money) but he was a psychotic murderer on top of it.
So many stories, so little time.

Ginger, for sure. But Nicky isn't far behind. They're both some of the most disloyal and disrespectful characters I've seen on film.

Killing and stealing much worse than adultery, NICKI !
the coins in the jar are for charity,
the coins in the tray are for sharing

Morally speaking, killing people and stealing is way worse than cheating on your husnband, being a bad mother and doing drugs.
Hpwever, I hated Ginger more, she was just a greedy beep

Ginger was definitely worse. Even though Nicky was a thief and murderer, Ginger was deceitful. She lied to her husband Ace, cheated on him, tied Amy (her and Ace's daughter) to the bedposts, wouldn't let go of her pimp/ex-boyfriend Lester Diamond, was greedy, drank, and did cocaine.
Avatar made on sp-studio.de

GINGER!!!! GINGER GINGER GINGER FCKING GINGER!!

It's hard to say really. Depending upon the context you're referring to I guess it would be difficult to choose. For one thing Ginger was a professional whore just out for money. Ace knew that and married her anyway, probably under the same illusion that many guys fall victim to where they think that they can change a woman over time. Wasn't going to happen with her and Ace should've used better judgment. As for Nicky, I thoroughly enjoyed watching his crew take care of him and his brother in the cornfield. He went from being a hot-headed, but reliable, enforcer and protector of the skim, to a doped up slob who made careless decisions (sleeping with Ginger/betraying his friend). I know it's just a movie and that the real life story had way more components involved. I'm just judging these characters basely solely on the movie itself. But, overall, I didn't shed any tears when Nicky or Ginger died.