I think the reason why Sakura always felt like she was kakashi's least favorite student was because she reminded him of rin. in the way she cares about her boys, and epsecielly in the way she always jump into danger for them. he was scared that she will die like rin, so he didn't want to get attached. but as we can see in this gif' as we saw in this episode' everything he said about Sakura, he clearly fail for he cares about her deeply, and if you ask me, she was his favorite all along