California

It is hard to say especially for business schools like Stanford or Harvard which are two of the world's most selective universities. Evaluations are done holistically and you must build your profile to fit into these schools. A poor performance in UG will not bar you from getting admitted into these schools if you have the right attitude. Yes, you must work very hard to outshine, a brilliant performance in your PG will not guarantee an admission, you will also require a quality work experience and demonstrate high-level leadership skills which again will not guarantee an admission unless you know why so badly you want to get into these schools and demonstrate exemplary potentials. Check their websites and understand the applicants they look out for. Prepare accordingly. All the best.