Honestly, 'The Fall of Gondolin' breaks my heart every time. It's Tolkien's foundational epic of the hidden Elven city's betrayal and ruin, written very early in his legendarium but carrying that raw mythic weight. The core is Tuor's journey, guided by Ulmo's visions, to warn King Turgon. You get this incredible build-up describing Gondolin's splendor—the seven gates, the gleaming towers—which makes the fall so much more devastating. The betrayal by Maeglin, lusting after Idril and coveting the throne, is such a classic, personal tragedy that opens the gates to Morgoth's armies. The actual battle sequences are insane, with dragons and balrogs wrecking everything. It ends with the desperate escape through the mountains, a real 'last hope' kind of exodus. What sticks with me is the sheer scale of loss; it's the end of a major Elven power in Beleriand, and it feels like it.
Reading it in the published Silmarillion version feels more streamlined, but Christopher Tolkien's standalone book, which includes the original draft and his commentary, shows how the story evolved. The early version has some wild differences, like mechanical dragons instead of the more organic ones. The central theme of hidden beauty inevitably discovered and destroyed by evil is just quintessential Tolkien tragedy.