I knew someone would mention Ella (junior), but she was eight when she died, so outside the rules.
Irene of Hesse and Heinrich of Prussia seem to have been well-liked by all their contemporaries (though Eric Lowe clearly doesn't like Irene!)
I'd not thought of Tatiana Konstantinovna, but she seems a good candidate. We could add her uncle Dimitri Konstantinovich (while no one has a GOOD word to say for Nikolai Konstantinovich). Apart from Nikolai, the Konstantinovichi seem to have been very pleasant people and well liked by everyone.
How about Gustav VI of Sweden and both the Connaught daughters? The only thing that could possibly be considered a bad word was Alice of Athlone's description of Patricia's paintings as 'Modern, very modern.'
Ann