''Good heavens! Whichever button did you press to make that happen?!''
Regardless of the ... that; no matter which.
According to or depending upon which.
''These chocolates all look delicious. Whichever shall I choose?''
Irrespective of the one(s) that; no matter which one(s).
Any or either one(s).
''I don't mind which you have. Take whichever.''
According to or depending upon which one(s).