I say Harley specifically because the shark joke came from the idea that sleeping with a shark is less crazy than her but also because she's my other all time favourite dc character and I think her and John should have freaky clown se*gunshots*
Potentially controversial opinion incoming: instead of the shark thing that's ruined Constantine's reputation in the wider dc fandom (and caused a lot of biphobia) I wish the movie just implied he and Harley hooked up once. That would have been neat