According to what?
Hayate never even comes into contact with Nishizawa in the anime, Nagi has spent more time with her than he has.
Hinagiku is far more likely, although he hasn't spent much time around her, she is the most popular character and she seems to be the only character other than Maria who he actually views as being female.
The anime seems to have spent most of it's time developing Hayate and Nagi's relationship, so it seems the most likely. Could easily be Hina, but it definitely won't be Ayumu.