They said that we should know who AD is by now so this suggests that they've been on-screen. The only way it could be Spencers twin is if he/she has been shown subliminally throughout the season, but would it still be Avery? If this isn't the case then it's definitely Ezra for the following reasons:
Stealing Emily's eggs
Generally messing up Alison's life even more
Breaking Spencers family apart (again)
Kidnapping and torturing Hannah
Aria.. nothing, apart from threatening her with a police report and letting her off scot-free and allowing her to leave the game?! No way
AND
I will think more shady things happened the night Charlotte was killed, it was confirmed a few of the girls left the hotel that night but I want detailed receipts. Also Hannah's mum was quick to get rid of the evidence
Melissa being AD would be good but it would make no sense to withdraw Aria from the game so she can sabotage her own parents - causing her mum to step down as state president.
There's so many unanswered questions
I still want more information on the NAT club, I read that the term "NAT" stands for "we see all" or something along those lines
And can we discuss Maya? Her death was so shady, no one knows how or why she died AND there's no footage of her death, did they even find her body?