Rahman, the use of th ein-tro music of the series Hercule Poirot in Ghajini songs was ingenious. If not for anything, I will hum ghajini songs, because the Poirot tune comes in now and then. I love that tune.
Answering this question is as difficult as trying to find the best of Kishore Kumar.
Rahaman is a Genius like none before him. Ever since "Chinna Chinna Aasai" and "Puthu Vellai Mazhai" through "Hai Rama yeh kya hua" through "Ghanan Ghanan" and "Mitwa" through "Aye hairate Aashiqui" through "Khwaja mere Khwaja" to the "Jai Ho" Rahaman has created history in melody and music.
It is impossible to list the best of Rahaman and sit back and realize some time later that... Oh I missed this one. Let us just enjoy the Magnificient Rahaman!