Romantically speaking? The only one that we know of for sure is Hancock.
Whether or not anything happens between her and Luffy though, I have no idea. The only way I personally see it ever actually happening though is if Oda were to play it out similar to how Akira Toriyama handled Goku and Chi-Chi's relationship in Dragon Ball, where Luffy doesn't quite "get" romance and sees Hancock as a particularly close friend who he just so happens to live with and engage in some funny activities when they're at home alone together.