i'd probably prefer to be a big fan of ai instead of patty and anna, but putting as much bias aside as possible i would probably say patty if we are talking only singles. i would base that solely on the fact that she has beaten like 16 of the current top 19 (with 4 three sets against justine, 1 versus venus, and 1 versus sharapova on grass and hasn't played sprem), has a GS semi and 2 quarters and a hell of a lot of 4th rounds, and has made some very big names looks foolish eg Mauresmo 6-2, 6-2 in Zurich last year, Myskina 6-1, 6-1 in Linz last year, Davenport 6-3, 6-2 and Dementieva 6-3, 6-1 in Charleston this year. she has the capacity to dominate almost any player given the right circumstances IMO, like she showed at RG last year when she won the 2nd against henin 6-2 playing flawless tennis. she beats anna i think because she is still a top player and within reach of a top ten ranking.
i can understand that some might disagree though. obviously if doubles is considered then she is blown out, but as jesper said she never played with the quality of partner as ai or anna. her doubles career is not bad though i have to say.