It is hard to choose between the different types. I also voted pop based on your examples but she is equally good at other styles.

She is great at singing ballads,her voice is fantastic in Final Distance and others of that kind. I would also like to hear her sing more funky songs like In My Room or Another Chance.

It's difficult to classify many of the songs, something like Hikari must surely stand alone in it's own genre of perfection!