I don't think I'd agree with Don's criteria here. Which is ok, because I think that threads which turn into lists of artistes run into the ground for me. (Especially when they are people I've never consciously heard in my life, which a good number of the ones mentined above are.)What I think is interesting is to work out what the qualities are that make you want to put a singer in a list.

If I was going to name one singer, it'd be Christy Moore. And the reason is the way he can open up meanings in a song you've heard before that you didn't realise were there, and make you want to sing it yourself. There's some singers put a mark on a song that seesm to make other people feel compelled to echo them when they sing it, and Christy doesn't do that. Yet he's got a way of singing that is unique - and I don't understand how those two things can be true at the same time.