Quick answer
No sé
the everyday version
Ni idea
the alternative — see below for when
Which one to use
No sé is the neutral default and fits every situation, formal or not. The accent matters: sé is the verb, se is a different word entirely.
Ni idea is informal and means you really have no clue. Among friends it is extremely common, but keep no sé for a boss or an official.
All the versions
No sé.I don’t know — neutral, always safe.
No lo sé.I don’t know it — with a specific thing in mind.
Ni idea.No idea — casual.
No tengo ni idea.I have no idea at all — stronger, still casual.
No estoy seguro.I’m not sure — softer, use <b>segura</b> if you are female.
Where you are matters
Both are universal, and in Mexico <b>quién sabe</b> and the shortened <b>sepa</b> are very common alternatives.
Check yourself
Which answer is the safest in a formal situation?