Could be a main game, or a Romhack, but for me my favorite final map has to be in Order of the Crimson Arm.
Many of the last maps I’ve seen take place in a temple or castle, and you’re fighting to save the world, (spoilers) whereas in OCA you’re fighting the last Braxians on the continent, as they make a final stand to buy time for their allies to escape. Regardless of what happens here, they’ve already lost.
The map theme is fantastic, Jormund had a pretty good motivation for invading the continent, and for better or worse, the chapter is brutal.
So brutal I had to cheese it by using the Gotoh as a barricade in the top right part of the map, which blocked off every generic enemy from attacking me. Despite that I had to do this rescue maneuver with Wayland, and use savestates so that he didn’t get one rounded by Jormund.