Who: Martina Hingis versus Anna Kournikova
When: During and after an exhibition match in Santiago, Chile (won by Kournikova, 6–4, 6–4), on November 28, 2000
What went down: After a dispute over a line call, Hingis made Kournikova cry on the court, declaring, “Do you think you are the queen? Because I am the real queen!” In the locker room, the players threw trophies and vases of flowers at each other.
What makes it special: (1) They were doubles partners at the time. (2) Kournikova actually won.
Just thought about sharing this, though I don't know how accurate is this. But according to GQ's web poll, this scores the highest with 46% amongst the other top 5 which includes:
Derek Jeter versus Alex Rodriguez
Martina Hingis versus Anna Kournikova (mentioned above)