The ending. Corvus reunites with Serena and is redeemed as they both Ascend to a Higher Plane of Existence, and the Celestrians finally ascend to the Realm of the Almighty, all while cheering and congratulating you. It then goes on to show the various people you've helped during the credits. Special mention goes to the implication that Marionette may be Not Quite Dead, Mayor Laria and Dr Phelming finally reconciling, Queen Voluptia being a friendly and just ruler after years of selfishness, the citizens of Wormwood Creek restoring their desecrated Guardian statue to its former glory, and the people of Upover placing a jug of Dragon Ale at Greygnarl's memorial as the thought-dead Greygnarl's shadow flies overhead.
Not to mention the immediately following side quest in port llaffan. The ghost of a woman who used to summon leviathen is reassured that her knowledge still has worth in modern times, Jonah receives her father's pendant from a second leviathan, thus keeping with village tradition, and then you are given a last fygg. The reunion of the Starflight crew is pretty darn heartwarming after the tearjerker separation at the end