singing is an interesting thing. theres people who are great singers that dont have good sounding voices (think dylan, or even bruce) and people who have great voices but arent technically good singers (think mariah carey--her phrasing is abysmal). now, there are a few singers that have good voices and are good technical singers--and these are the people that there is general agreement on. you make a list of these people as great singers and you generally dont get much argument. marvin gaye, elvis, paul mccartney, david ruffin of the temptations, kate bush, (to get out of rock a bit) sinatra. thats a few of them. anyone that tries to tell you that marvin gaye couldnt sing doesnt know anything about singing.
