Most likely to be A.D. ?

Forget theories*

I see it going one of two ways. She either chose to go with one of the most popular fan theories, (in which case it will be Aria, Wren/Melissa, or Alison) so she can feel better about the way the CeCe reveal went down.

OR

It ends up being some rando who we don't know or care about. Like maybe some guy Spencer accidentally bumped into one time and he was hellbent on destroying her life and the lives of people she loved.

The world isn't split into good people and Death Eaters.

It should be Melissa so I'm hoping for her. I've been wanting Melissa to be A since season 1.

"When life gives you lemons"
Jessica D: sleep with their fathers and have secret lemon children 

Since they seem to have been fighting hard to get Torrey Devito back I have to assume that it's Melissa or that she at least has something to do with it.

I have a feeling she's going to go the Lucas route but that's just me

Wren + Melissa or Lucas.

I'd say Melissa or Lucas. They're clearly not willing to break up any of their precious endgame OTP relationships, so all of the Liars and love interests (Toby, Caleb, Ezra, Alison) are out. I like to think that they wouldn't be stupid enough to make the only living character with an established mental illness the big villain again, so hopefully Mona is out as well. Charlie Craig confirmed that Wren is involved in a big twist in the finale, and I assume that if he was AD that wouldn't have been given away.

Aside from the side characters and the obvious shadies (Mary, Jenna, etc.), who else is left?

Melissa or Lucas