Sixties. It was a period when you had singers like Rafi Sahab, Lata ji, Mukesh ji, Manna dey, Hemant Da etc , along with great music composers like Naushad , Lakshmikant Pyarelal, OP Nayyar, Shankar Jaikishan, Roshan, Madan Mohan etc. This combination of great singer-Composer was complimented by unforgettable lyrics by Sahir, Kaifi Azmi, Anand Bakshi, Shailendra etc. Even some of the great movie makers of that era ( like K Asif, Raj Kapoor, J. Omprakash, Shakti Samant, Raj Khosla, Manoj Kumar etc, ) had an excellent ear for music. All in all, it was a golden period as far as melodious music in hindi films is concerned.
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I agee with Rajendra Jerath's answer but would like to add that 50's also had very melodious songs .The best examples are Raj Kapoor's Awara, Barsaat,Shree 420and the compositions of Husnalal Bhagatram in Bari Behen etc.