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"Ничего себе" is used in positive/negative situations equally.
"Вот это да" is rarely used in negative situations, however it can be used with proper tone.

It depends on the context and surrounding phrases. All of these can be best choices.

It has very wide spectre of meanings and emotions.
"wow", "I understand you", "yep", "amazing", "disgusting", etc. as meanings.
It is used more often with negative emotions: sorrow, distrust,...