"Darnedest" is a synonym for "damnest" that is commonly used. It is an informal and slightly less offensive alternative. Other synonyms include "most confounding," "most ridiculous," and "most absurd." These words can be used to describe puzzling or frustrating situations. Another synonym is "most unusual," which is used to describe something out of the ordinary. "Most surprising" is another alternative that can be used to describe unexpected developments. There are a variety of synonyms that can be used for "damnest" depending on context, but these are a few of the most common options.