Bittersweet endings are the best. Ambiguous endings are (with rare exceptions) the worst. When I play a game/read a book/watch a movie, I want clear resolution unless it's known there's going to be a sequel beforehand.
FFVI spoilers
I know this has been touched on already, but Kefka absolutely did not win. He clearly stated (before the final boss fight, in fact) that he wanted to destroy people's dreams and their hope, and he could NEVER do it, no matter how many towns he vaporized with the Light of Judgment. And that's what FFVI is all about. Even when things are awful, don't give up hope.
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