She held down the Kyuubi after giving birth and having the Kyuubi yank out of her . . . . . :|Tsunade and Sakura are just the start.Kushina holding down Kurama with her chakra chains for a were minutes is not a feat worthy of putting her among the top kunoichi..
A fellow female junchiriki in Yugito has shown better jutsus even though it was after her death.
Mei has two kekkei Genkais and has master three element natures along with being the Mizukage.
Pakura has a kekkei Genkai has master at least two chakra natures and can literally turn an enemy to a mummy by just running by them.
Tsunade - This is not even a question.Feats alone put Tsunade on a far different playing field then Kushina.