I know many people will be surprised by my answer but personnally i would say that she played her best tennis in Sydney in 2000 when she beat consecutively Pierce,Hingis and Davenport without losing a set.
More than the results themselves,it was the way she was playing in those 3 matches(especially against Davenport) which was unbelievable: she was playing so agressively(much more than after 2001) and was hitting winners from both wings(her FH was much more penetrating back then) and almost everytime.
I know she won much bigger titles than that few years after but to me,it was the tournament where she probably played her best tennis,especially from the baseline.
More than the results themselves,it was the way she was playing in those 3 matches(especially against Davenport) which was unbelievable: she was playing so agressively(much more than after 2001) and was hitting winners from both wings(her FH was much more penetrating back then) and almost everytime.
I know she won much bigger titles than that few years after but to me,it was the tournament where she probably played her best tennis,especially from the baseline.