At first I thought this would be a tough choice, but then I realized, no, it wouldn't. I love all my dolls for different reasons, but the only one that would be impossible to replace is Rune. I thought maybe Seona, because he has the most work put into him... but if it was necessary, I could recreate him. My other dolls are basic, unlimited sculpts, and even though it would break my heart to replace them, and the cost of Shy has doubled, so replacing him would make me cry, it could be done.
Technically, Rune could be replaced too. He's not an unlimited sculpt, and I could probably redo his ear mod. But he was an accident with a happy ending, and there's just too much sentiment attached to him. His birthday recently passed, too.