I think I know who A is!!!
I only have one piece of evidence to support this but...
MONA. You know how at the end of the episodes it shows a leather-gloved hand doing something? Well in one, it showed A purchasing riding boots.In "Touched by an A-ngel" Mona and Hanna go riding with Kate!!! A couldn't be Kate because she is familiar with riding and would most likely already have a pair of riding boots. Also Mona had reason because Ali, Aria, Spencer, Hanna, and Emily all treated her badly.
Ever since I've realised this, I decided to re-watch PLL. I've picked up some other things that connects A to Mona.
1. When Ali and Aria are walking with the fro-yo, Mona is chasing them. They ignore her and walk around the corner to find Byron kissing Meredith. Mona could've seen and taunted Aria as A in the future.
2. When Hanna is in her car, the radio comes on. A dedicates a song to her with the message: "To Hanna, from you best friend. -A" Mona is Hanna's best friend!!!
3. At one point Mona tells Lucas to ask the wizard to... "make him a man." Some following messages from A are: "Ding dong, the bitch is dead." and "There is no place like homecoming." Those are all references towards The Wizard of Oz.
These could just be theories that I'm over thinking, but it really makes sense. Please read!
I dont have any proof, or clips to back this up, but I think "A" is Paige.