Former, friend, someone I had a special relationship with. Some will take more evolution that others to become a warm memory, but I think most achieve that. Your point is well taken that we gain good things from most relationships. Happily ever after doesnt have to mean we ended up together, and it can still be true.
Alternatively, for one who inflicted a hurt so deep that you think you might never recover, the term ex seems not enough. Emotional terrorist? "Oh, look, there's my emotional terrorist. We're not cool yet, but maybe someday." Hide the sporks!