Pandit Bhimsen Joshi (1922-2011) was one of India's most celebrated classical vocalists and was often referred to as a 'maestro'. He was a legendary exponent of the Kirana gharana and received the Bharat Ratna. While Hariprasad Chaurasia (flute), Nusrat Fateh Ali Khan (Qawwali), and Bismillah Khan (shehnai) were all legendary musicians, Bhimsen Joshi was most commonly honored with the title 'maestro' in the context of this question.