Oh, I think this is absolutely a matter of personal taste. There is no yardstick here, except for maybe the ability to stay on key...and even then, for certain genres of music, it's thoroughly unnecessary. Much extreme Heavy Metal has no actual melody at all, just a lot of screaming...yet there are those who have their "favorites" there as well.

I could tell you who I think is a great singer, but that would only tell you who I like, not who is actually a better singer than anyone else.

For me, as you mentioned, the ability to emote seems more important than training and absolute control. But then again, my Father might disagree with this, being the classical music fan that he is. So even there, it is subjective.

So, there ya go! No answer at all from me, really. ;D