Since I am one of the Tolkien fans who loves the world of Middle Earth as a whole over the major storylines,
Unfinished Tales is especially beloved to me.. I know its fascination is dependent on having read the "Finished" works but it answers so many unanswered questions that the appendices could not - or more accurately offers answers since the nature of it means that they cannot be taken as definitive. So hard to choose - I love getting Gandalf's view of the Quest of Erebor, more information on the disaster of the Gladden Fields, the Lords of Dol Amroth, Numenore ..I couold more or less list the index - however the Tale of Aldarion and Erendis is perhaps what I would go for.... far more passion than the whole of the Lord of the Rings and a psychologically accurate study of a disintegrating relationship.... tree loving heir to the throne hitches up with beautiful but neurotic and wilful woman .....