For my money, it has to be Children of the Storm. So many wonderful feelings associated with that novel. To me, it embodied the Star Trek philosophy to its core.
This for me, too.
Don't get me wrong, Full Circle is a hell of a book, and Eternal Tide is an even more hell of a book (or something...they're both great, is what I'm saying), but CotS really hit the philosophy of Trek perfectly. It's as perfect an encapsulation I've ever seen of why I'm a fan of Star Trek in the first place, with some of Kirsten's most intense and most beautiful writing. Killer novel.