How can anyone deny itsuki ending at any point? harem manga always work exactly the same girl: first girl introduced wins. End of story. From that point onards, any dieas one may get about the other girls having a chance is pure delusion.
The only other option I'd accept as reasonable is the harem ending, due to the whole quintuplets thing, but it's still a minimal chance. It's a shame, because Itsuki is worst girl, but she will win.