undoubtedly it is A.R.Rahman. if you dont want to go by the popularity, then go by what the contemporary competetors have to say. from Himesh Reshmiya to Shankar Ehsan Loy, everybody's first choice is A.R.Rahman. One more thing we should consider, after Pandit Ravi Shankar, Rahman is the person who took Indian music into world map.
Based on the number of hit songs given a close competitor for A.R Rehman will be Himesh Reshmaiyya. But nowadays if a new song comes you can easily makeout if music director is himesh reshmaiyya. That shows that the he is giving all similar kind of tones, this gives a negative edge for Himesh as compared to A.R Rehman. Rest all music directors cannot compete with these two.
Its really a tough choice to make but in any case AR Rehman has to be the no. 1...the variety of music (Lagan, Swades,recent blockbuster Rang de basanti and many more to mention)he has given till now is something fabulous stuff. So in my opinion AR Rehman is the best music director of Bollywood.
there is only one genius and thats a ra rahman. i will expalin with examples-
1. none could have thought of a son like "luka chupi" in RDB at son's funeral from a mother. the director ( rakesh mehra) has acknowledged it was only rehman's idea to have a song there.
2. the whole musical score of "Dil Se". the breathtaking title track of dil se, the awsome chaiyya-chaiyya, the soul stiring ae ajnabi and the brilliant satrangi re.
3. only rehman could have songs like telephone dhun, maya machindram, patti rap, rukmani rukmani and get away with it in flying colours.
4. Connoisseurs of great music are still to get over the divine experience of his score in Roja,Bombay, Taal and Zubeida.
5. his revolutionary interpretation of Vande Matram and the periodic purity of Lagaan took the whole nation with storm.
6. if ever fun was fun than it was in his score in Rangeela, Yuva, Gentleman, Jeans, RDB, Chandrelekha, Boys and Sapney.
7.. finally to listen rahman is not about listenig to music but a way to connect to the GOD. its a spiritual journey everyone craves for.
one final word don't even mention himesh reshammiya in the same breath with Rehman.
