This one is a really interesting battle because I see it going both ways.
Power Ranking
Logically, Zeref (FH) should take this. He is immortal (cannot die) and has infinite magic power (unlimited attacks). Zeref is virtually a god at this point. Unless Acnologia can somehow extract this power from Zeref and use it for himself, he loses.
Reality (Prediction)
Despite being wrong many times, I predict that this fight will be canon. And more than likely, Acnologia will win. Working backwards, we know Natsu will be the one to defeat Acnologia. And it seems he's already shown his last trump card. So the only way for Natsu to win against Acnologia is to have Fairy Heart's power. And for that to happen, Zeref will most likely have to die. The way I see it, Acnologia will kill Zeref and Natsu will attain FH to avenge his brother. The real question here is how Acnologia can bypass immortality. And I really don't know how...