Kishore, Rafi and Manna Dey.

Salil Chowdhury used to feel that Manna Dey was not given due respect in the hindi film industry. In spite of singing for Raj Kapoor (in fact most of his hit songs were sung by Manna da) it was Mukesh who was known as voice of RK. Also the group of Rafi and Naushad were experts in churning out hits, and were at their peak. Salil Chowdhury has said that he felt that Manna De was sidelined as he was a Bengali and the music directors felt he would not get the nuances of urdu correctly and so naturally felt an animosity towards Rafi, but in a concert, Manna De said publicly that Rafi was the greatest.
As for Rafi he once said everybody listen to my song but I listen to Mannadey's songs.

Salil Chowdhury felt that Mohammad Rafi tended, unwittingly, to indulge in vocal mannerisms.
He was not the only one who felt that way.

During the recording of Jaane Kya Dhoondte Phirti Aankhen Tujh Main (Shola Aur Shabnam), Rafi needed over 20 takes. Even then (composer) Khayyam was not satisfied.

But that could be Rafi's off day. This is the only industry where you cannot have off days.

Rafi could not get a particular portion of Tasveer Teri Dil Mein (Maaya) right as well, and Salil Chowdhury was trying to explain it to him.

After many takes, Lata Mangeshkar got angry with Rafi and snapped at him. The thing with Rafi was that if you got very demanding, he would be put off.

O P Nayyar and Naushad would not be demanding with Rafi because they understood him. But that day, Rafi got very upset because Salil took Lata's side.

Salil always considered Rafi to be just an okay singer

As for RDB, he always felt in awe of Rafi as he was the chosen singer for his father SD Burman, and when RDB came onto the film scene on his own, he wanted to create a distinct identity rather than being known as a clone of his father, and hence chose Kishore Kumar who was a close friend of Mehmood who was RDB's closest friend. He also changed most of the orchestration to sound different, and providentially his songs clicked becoz of the change, and Kishore Kumar had arrived.

However RDB admitted in his last interview immediately after 1942 a love story, where he said that Kishore Kumar was his favorite, but any song that Kishore sang, Rafi could sing, but not all that Rafi sang Kishore could, eg. classical.

Kishore himself said that I can sing song in one way but Rafi can sing the same song in 100 ways.

But that one way of Kishore was awesome in most of the cases.

Kishore's voice was most powerful and would overcome other singers. It was sheer confidence and natural talent of Kishore which overshadowed MannaDey in ek chatur naar from padosan.

Only Rafi could stand infront of kishore in duet song like hum premi, yaadon ki baraat, salamat rahe dostana, yum na sharma, etc.