I know anything related to PLL can be a little annoying for some people, so I’m going to list my reasons after the jump. 

• Aria is “A”
• She is the only character who makes the “shh” motion in the opening credits, advertising photos, etc. I used to think that maybe this meant that Aria had the biggest role in the show, but it’s become clear that the girls’ roles are equally divided. 
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• Aria was away in Iceland for about a year and easily could have been planning the entire scheme while she was gone. After all, the text messages from “A” didn’t start until Aria returned. 
• Everything that “A” makes Aria do usually ends up benefiting her (ie: getting rid of Ezra’s ex)
• “A” told Aria’s mother about her father’s affair through a letter. It would be much easier for Aria to anonymously tell her mother about her father’s affair compared to telling her face to face (and potentially damaging her relationship with her father). 
• Aria could have planned to ditch the dinner with Ezra the night that her father received a letter from “A” to that they would be at that particular restaurant. If Aria’s father didn’t catch Ezra and Aria there, he would be under the impression that they weren’t together anymore and she wouldn’t have to sneak around as much.
• “A” is often seen using a typewriter. Ezra owns a typewriter and Aria has a key to his apartment. She could easily sneak in to use his typewriter and also could have been the one who stole his diploma. 
• Aria enjoys both English and writing. This is only a small detail, but could easily connect her to the letters that “A” often sends. 
• The writers/producers claimed that “A” would be dressed as the Black Swan at the ball. I believe “A” made Melissa dress up as the Black Swan to serve as a distraction from the real Black Swan. 
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• In that same episode (UnmAsked), Aria was the only one that took her mask off. I believe the producers said something about the “unmasking of A”. 
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• Aria was the one that shouted for Hannah to stop the car when she almost hit Mona. It honestly seemed like in that moment she needed Mona to stay alive (because Mona was working for her). 
• Aria was the last person with Vivian Darkbloom’s red coat. Somebody wearing that same red coat visited Mona at the end of season two, and it was clear by that episode that they were “A”, and that Mona was working for them.
• Mona once called Aria “Big A”. I distinctly remember Aria freaking out a bit, but everybody seemed to have thought it was because Mona was talking to her. Mona talked to the girls quite a bit at school, so I don’t think that justifies Aria’s look of surprise. I think she was shocked to hear Mona call her “A” in public. 
• Aria is always referred to as the best liar in the group. She even admitted to being “A” once to Hannah’s mom. I know it was for the sake of Hannah, but the producers have been known to give us blatantly obvious clues that they know we’ll overlook. 
• “Miss Aria You’re AKiller, Not Ezra’s Wife.” I know that this spells out “Maya Knew”, but I can’t help to think that there’s a double meaning. It also was the only phrase that the girl’s didn’t decode at the end of the last episode. 
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• ^In my opinion, this scene was a clear indication that Mona was trying to reveal Aria as “A”. 
• Apparently in the last episode you can pause the scene where Aria and Hannah are sleeping in Aria’s room and see two books on her bookshelf about killing and death. Haven’t seen this yet, but I really wouldn’t doubt it. 

As far as motives go, I am still tossing around a couple theories. I’m trying to find connections between Aria’s relationship with Ezra and “Lolita”, the book that Alison showed Aria. I know that the writers tried to make Aria out to be the cliche weird one of the group in the pilot (ie: pink hair, always wore black, etc.), so I’m wondering if maybe being “A” is a way of feeding her ego and helping her feel more confident, etc. I’m not entirely sure yet. 

ALSO, WHAT IF…. 

A TEAM = Aria, Toby, Ezra, Ali, and Mona. This literally just came to me five seconds ago. I might be insane.