Do you mean best technically, or favourite? Most people will answer the latter, even when the question asks the former. I couldn't begin to answer either (except that Michael Jackson would be a loooooong way off being my choice under either category).
It has to be Cliff Richard who according to the Channel 4 tv program ultimate pop star was number one for selling the most records in Uk.
His next single out on Monday, Thank you for a lifetime looks like being another number one.
Michael Jackson has made some very good records and I agree he is also good on stage.
If you are saying of all time you can't forget Matt Munro who had a great voice.
I wouldn't say Michael Jackson is the best singer of all time, but certainly the greatest pop star for great songs and great music. It also depends for what category of music, some would say Elvis for Rock & Roll or Marvin Gaye for Motown.