Cleyra, Lindblum, or Alexandria? I guess you can count Terra too if you want, but I mean destroyed by eidolons.
The Lindblum scene is kind of sad because of how emotional Garnet is, but the Cleyra scene does it for me. At least Lindblum has a chance to rebuild, but Cleyra is completely destroyed (which is also sad because it's an awesome location) and most of its inhabitants die.
I remember the first time I played this game, I had no idea what was about to happen...I really thought they were going to save Cleyra! Nope.