I've only read Unlocked, Leaving, Learning, and half of Like Dandelion Dust, and I 愛 allll of them, but my お気に入り would have to be Unlocked :D It's such a great story :)
It's a very hard choice between the Firstborn series, Just Beyond the Clouds and Beyond Tuesday Morning, but I'm going to have to go with Family, the 秒 last book in the Firstborn series. It just has everything...Dayne's growing relationship with both his earthly and heavenly father, Ashley finding out about him being her brother, and his proposal to Katy!!