I definitely think the one she'll go after is Snow White! Regina may have become Snow White's friend, but Snow White is the one that the Evil Queen hates the most. Plus, with Regina having her darkness taken from her, she likely doesn't have the same kick to her that she had before. Emma is also the one I think she's likely to go after first! I'd say she probably hates Emma and Snow White equally, she just has more of a history with Snow White. Although, if she tried to kill Emma, it would hurt Snow because she'd have killed her child. Plus, Emma is the one who ruined her happy ending of making everyone suffer from the curse. She's also likely go after David and Killian, but it's unlikely they'd be the first she'd go after. She'd just want to hurt them because their Snow and Emma's true loves. Plus, she has a history with David and hates his constant whining, as said in the episode where Regina is effected by the cursed mirror back from the Frozen story arc. I'm not sure how the Evil Queen would feel about Henry or Zelena. I mean, she is supposed to be pure evil (I assume), so I'm not sure if she loves Henry or what kind of feelings she'd have for him. I'm not sure how she'd react to Zelena, that should be interesting though. It's also possible she'd go after Belle (still asleep) in order to get something from Rumple. I'm not sure how she'd react to Regina, though again, it should be interesting. She'd probably see her as too weak to be a threat to her, now that she's lost her darkness.

Either Regina, Snow, or Emma. I feel like her going after Regina would be the logical move, if she takes her out then she could shoot for impersonation and she could get everyone easier because they trust Regina. Plus Regina tried to murder her.

I just feel like she has this super long vendetta with Snow White that she has become obsessed with and I do think she'd want to impersonate Regina to kill Snow now that she trusts her. At the same time, she could want to go after Emma for "stealing" Henry and basically being the one who made Regina reject the Evil Queen.

I think it will be more of a personality battle.

^