Hancock probably
I don't think Luffy would ever fall in love with a woman, but I think he could have a kid with hancock as hancock needs to have kids to help keep the kuja tribe alive and I feel like Luffy would do Hancock a solid and be the dad, just because he knows how much hancock cares about him.
Especially since if he has kids with a kuja, he won't ever have to take care of them, meaning he can just be an absentee dad sailing the seas forever
I don't think Luffy would ever fall in love with a woman, but I think he could have a kid with hancock as hancock needs to have kids to help keep the kuja tribe alive and I feel like Luffy would do Hancock a solid and be the dad, just because he knows how much hancock cares about him.
Especially since if he has kids with a kuja, he won't ever have to take care of them, meaning he can just be an absentee dad sailing the seas forever