Answer by Sasi Inguva:
Lukachuppi from Rang De Basanti. To give you the context, the son of the old woman, who was an Indian Air Force pilot, had just died while flying a defectively manufactured air plane. The lyrics of the song describe the mother searching for her son in his childhood and getting worried about him, while the child was roaming somewhere flying kites and dreaming about becoming a pilot.
I got goosebumps and tears, when the last part of the song started – when the airforce officers come to the mother's house to give his uniform back, while AR Rahman's voice in the background rises to a high pitch.
I forgot to mention one more song, which is as equally emotional and moving as the above song, if not more. That is Tere liye from Veer Zaara. (Spoiler Alert) The man, an Indian, had been serving a sentence for life in Pakistan for the past 20 years, to protect the pride of the woman, his lover and a Pakistani, believing her to be married to someone else and leading a happy life. But the woman, believing him to be dead, breaks off her marriage and runs off to the hero's house in an Indian village, setting up a school in the house to teach village kids, which was his father's dream and lives there since then for 20 years. Finally they meet each other in this song.
Gulzar's lyrics describe their turmoils of being apart, so poetically that they will cringe your heart. The voices of Lata Mangeshkar and Roop Kumar Rathod, penetrate the deepest corners of your memories. Of equal prominence the picturization of the song, the genius of Mr. Yash Chopra, which shows them looking at each other but perceiving the younger versions of each other.