I think that A is someone to hurt each person. Hanna=Mona. Spencer=Toby??? Aria=Meredith/Ezra (can't really think of a motive for him). Emily=Maya. Mona we already know. Maya, we just learnt, kept a whole bunch of Ali's old stuff. Suspicious? Meredith and Aria kinda hate each other. Ezra, I don't think he would have any reason to and he was one of the things Aria was being blackmailed about, so I doubt it. Toby might hate them for teasing him and getting him sent away, but he seems like a nice person. So did Mona. I'm not sure of anything of course, but those are my theories.
I also think that Ali killed her twin for some reason and is still alive, leading the A team. What her motive to hurt these girls is, I have no idea. She did, however, visit Mona, as we now know. Maybe she found out that the police were going to examine the body closer and that they would possibly find that it wasn't Ali, so she took the body.
Ali has/had a twin. In her room, there is a picture of her, Jason, and another of her. Therefore, at one time, "Vivian" also had blond hair. Hmm. The twin could also have killed Ali and removed the body so that the killer, whoever it was, wasn't revealed. The killer wasnt Garrett. I think they're misleading us. I don't think they would have told us who it was so soon if it was. But what about Ali's parents? If there was a twin, which there almost certainly is, wouldn't they have mentioned her sometime? What if they're a part of this? For some reason, I doubt it. But if they killed Ali for some reason and kept Vivian a secret…
Maya. She really has become an issue. So what if for some reason she was part of the A team. Why was she killed? Maybe she was going to tell someone who was doing it? She could hate Emily because of the Trista thing.
What do you think? These were all guesses based on random things, so please don't rely on these, but do tell me your opinions.