"Jupiter" is her biggest hit so I guess start with that song.
Some of my personal faves:
Ashita (明日) (From the album ODYSSY)
Hoshi Tsumugi no Uta (星つむぎの歌) (From the album Path of Independence)
Siciliana (シチリアーナ) (From the album My Classics!)
Inochi no Namae (いのちの名前) (From the album From To, it's a collab with Joe Hisaishi)
Greensleeves (From the album My Classics 3)
NOT A LOVE SONG (From the album Doki!)