I think you already know my answer but oh, well I'll repeat again Tannu is way more evil than Aaliyah. Not just because she slapped Tannu. But for other things as well. From the beginning I've preferred Aaliyah than Tannu,
Here are few reasons.
When Aaliyah was still under the assumption that Pragya is the woman that Purab is in love with,
1. She stopped Abhi when he raised his hands kind of to strangle Pragya. She calmed him down.
2. When she placed the engagement ring in Pragya's purse, she straightforwardly told Abhi that she was the one who hid the ring, whereas Tannu conveniently left out that part and merely told him that Pragya took the ring.
3.Later, when Abhi raised his hand the second time, he stopped himself but Aaliyah later pushed him away, and talked to Pragya much calmer. And Tannu was the one smiling in satisfaction, in spite knowing the truth that Pragya was innocent.
4. And during the episode where Pragya restricted Abhi from drinking, he said that he's going to get drunk and hit her purposely, Aaliyah asked him to stop thinking such foolish ideas.
5. It was Tannu who wanted Abhi to slap Pragya, not Aaliyah.
6. Aaliyah is still under the assumption that Pragya helped Purab and Bulbul run away, while Tannu knows the complete truth after reading the letter.
7. Aaliyah maybe a spoilt little brat, selfish, individualistic person, but there is still hope for her because she loves Abhi and Daadi.
Recently, her behavior disappoints me most definitely! I am not happy that she left her brother to stay in the cell and also framing Pragya on such a huge accusation. But she isn't perfect, just like almost all the characters in KKB! She doesn't want Abhi to fall for Pragya and she wants her out for revenge. She believes she has every reason to hate Pragya.
Aaliyah is this spoiled girl who always gets things her way and acts out when she doesn't get, them.
Tannu maybe an outsider but she is the one who loves only herself and wants Abhi only because he's slipping away from her hand. She doesn't care about Abhi or Daadi.