"Best girl is the traitor"

Which one of them?

I think it's Tooru Hagakure:

-She's had no real character development over the other girls, even the less focused ones like Jirou have had more screentime.

-We've never seen her fight, not once, not during USJ, not during the sports festival, not during the final exams, the summer camp is a pass because she was knocked out during that, but even then, she didn't do any major fighting during the provisional licence exam arc.

-During the class rep voting, only 19 votes out of 20 were cast, 16 were shown on the board, but only three were given to other people: Ida and Ochako gave theirs to Deku and Todoroki gave his to Momo. The only one missing is Hagakure. This wouldn't be a problem if it weren't for the fact on the same day, UA was breached and the class schedule fell in villain hands. Hagakure would be the only one to know UA's layout to conduct a raid and the only one to stay invisible.

-During USJ, she says she was with Todoroki, but somehow knew Ojiro was fighting alone. And Todoroki froze the entire arena and she wasn't stuck. It's likely she was with Ojiro but stayed hidden so he would be cornered and killed.

-When Deku was snuck up on and interrogated by Shigaraki on the shopping trip, how did Shigaraki know where Deku would be at the exact spot? Who was the one who suggested they go on the shopping trip? Only Hagakure.

These things could just be coincidences, and we have no hard evidence, but that's my strongest guess.