I think Navratilova.
I recall a comment made by Chris Evert, saying that Martina's best was better than hers and Martina's worst wasnt as bad as her worst.
I just think in terms of dominance, no-one can have good days every day, so my vote is Navratilova.
"Tennis is just chess with balls....zugzwang"
"The older I get....the better I was"