Best defender? Going back one would have to consider Esoleta Whaley (spelling is likely wrong), Annette Smith Knight, Beverly Williams, Clarissa Davis-Wrightsil, Ellen Bayer, Retha Swindell. There are others I don't remember who had defense as their strong suit. A look in the media guide will surely bring their names to the forefront and point out several I should have mentioned, like Andrea Lloyd-Currie who was the best all around freshman I have seen at UT. She walked on campus with a complete game. Her defense was all over the court.
Have I named the best defensive player? Its likely. Can I say which one was the best? No way. I have seen those named play remarkable defense, and some also played remarkable offense. I think that many would agree with whoever I would name, just as I agree with those already named. Do I have a favorite defensive game? Sure. Ellen Bayer blocking TT 12 times in one game, 4 of which in one sequence. She blocked TT's high scoring post player so many times that the woman passed the ball out from the paint rather than get blocked again.
Yulanda Wimbish at 5'9" (close) just shut down 6'4" Sue Wicks by face guarding her in a game at a tournament (1987) in Miami. Wicks became so frustrated that at one point she just stopped and screamed, "What are you doing?"
There has been no one who could anticipate a pass for a steal or roam the paint better than Annette Smith Knight although Raven is getting better at steals. Lots better.
I will have to say that we have had numerous examples of very good defensive players and I feel lucky to have seen some of the best at their best.