Interesting one. Got me thinking hard. Are we talking about the quality of the voice and the range both of notes they could hit and styles they could sing?
For me Mercury was stuck too much in a genre I grew out of which perhaps makes it hard for me to consider him that great. did he ever really get out of that genre bar a bit of mock operatics?
Bowie an astonishing output over the years of incredible inventiveness but never anything more than pop/rock songs really of a similar type? Thought a bit more – some that is really soul and some funk as well. The first white man to appear on soul train in the US
Boy George had a beautiful voice at his peak. Could Bowie sing a love song as sweetly? His voice is gone now unfortunately
Some wonderful pure voices amongst the reggae stars but the best of them are Jamaican not British despite spending much of their career in the UK or I’d have someone like Horace Andy or Beres Hammond
Others with character in their voices like Lee Brilleaux but not the range of those mentioned and definitely only one genre
Nope – I’ll agree with Bowie for musical ability x range of styles x range of notes. Others might beat him in one area but not all three combined