Gloomhaven

In cases of ambiguity where players decide the outcome, how should the players decide? Should they do what is best for themselves or worst for themselves?

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Up to you
It is up to you. That is why you are deciding. It is assumed that you would decide whatever is best for you, but, again, it is up to you. Note that ambiguity with respect to attack modifier cards drawn for advantage/disadvantage are not decided by the players - the rule book states how to resolve that (use the first drawn card).