Currently Modi is the Prime Minister of India which elects its PM every 5 years. When Modi stops being the Prime Minister of India, who will replace him?

In case of a death of a Prime Minister, an interim PM is appointed for a few days until the Parliament selects a new full-time Prime Minister. The aim of this question is to guage who is the person who is most like to follow Modi as the full-time PM of India, so any person appointed temporarily for a few days won't count.