If you want to be emphatic, "non ho la più pallida idea" absolutely works. For example if someone asks you where you are and you are completely lost, this would be appropriate, especially if you are exasperated. "I have absolutely no clue"...really "I haven't the slightest idea"
Non ne ho idea
Non saprei dire
You could also just say "Boh" which is spoken and very informal, but means "I dunno"
Chissà is "who knows"
What time are your parents coming home? "Chissà" (who knows)