Most captivating: Rhovannion in general and specifically
both the various polities of elves, men, and dwarves, and
how they seemed to generally ineract in such harmony (Hollin
on a grand scale).
Most mysterious: The area of the Sea of Rhun. Who lived
there at various times, what was it like, how might JRRT
have utilized it in other tales of Middle-earth, (how was
wine made there
), etc. Of course, as Tolkien observed,
that's part of the interest of Middle-earth, barely glimpsed areas,
vistas, stories.