While there are many ways to say this correctly, the closest one to the tone and style, if you are referring to the "sugar" version of the phrase, might be
NPI.
If you are looking for something colloquial, but a lot less 'informal', you might try, ¿Yo qué sé? or its companion ¿Qué sé yo? Of course there is 'No tengo ni la más mínima idea.....'