Pretty Little Liars had a ton of twists and turns throughout the seven seasons that it aired. The big question on every viewer's mind throughout the entire series was who exactly could be behind the anonymous messages that the main characters were receiving. Who is "A"? Why are they doing this? These are things that the Liars and the viewers wanted to know and throughout the series, there were quite a few possibilities for who might have been sending these messages and threatening to ruin the lives of the main characters.
It seemed like every few episodes, the Liars had a new suspect for who might be the person behind the "A" messages they were receiving. Watching the show, there are quite a few suspects that the Liars had their eyes on that seemed like pretty good candidates for being the person who was leading the entire thing. They had motive, they were definitely creepy, and there were a ton of clues that lead many viewers to believe that it might have been them. And then, there were some people that the Liars suspected, if only for a short period of time, that viewers could tell they just had the wrong idea about.
A-Team Material: Aria Montgomery
Despite being one of the Liars and one of the main characters of the series, Aria Montgomery was definitely a popular choice for theories about who "A" might be - and not just because of her first initial! There were a lot of clues that pointed fans in this direction, including future A-Team reveal Mona Vanderwaal greeting Aria as "Big A" at one point in the school hallway. Having Aria be exposed for being the person that was tormenting her friends and the people in her life would've been a pretty interesting twist.
Total Red Herring: Toby Cavanaugh
Toby Cavanaugh is a character that was suspected to be a "A" in the early seasons of the series. He's the step-brother to Jenna Marshall, he has a bad reputation around town, and he even has a tattoo of the date that Alison went missing. Despite the fact that Toby has all these questionable attributes, it's shown fairly early on that he's simply misunderstood and he comes around. Though he worked with "A" for some time later on in the show, most of the things that made Toby seem like he might be up to no good were simple coincidences.
A-Team Material: Melissa Hastings
Melissa Hastings is the older sister of Spencer Hastings, one of the main characters of the series. Although initially, it seems like Melissa's only real problem is with Spencer, it becomes clear in later episodes of the series that there's a lot more going on with Melissa than a simple sisterly rivalry.
From ties to the N.A.T. Club to some intense drama with Spencer, Melissa has a lot going on that make it easy to believe she could've been "A" all along. Alison knew that Spencer had a penchant for getting too friendly with Melissa's boyfriends, so it's not a stretch to say Melissa may have lashed out at Spencer and her friends. Plus, this would be much more interesting than inventing a brand new sister for Spencer to be A.D.
Total Red Herring: Byron Montgomery
Byron Montgomery is Aria's father and he wasn't actually an "A" suspect for very long. Although he would have had motive - Alison knew that Aria saw him having an affair which ultimately ruined his marriage and temporarily tore his family apart, it simply doesn't make sense. The Liars suspected that Byron might be "A" when Aria went missing for a short period of time, but the idea that this man would spend so much time tormenting a group of teen girls just seems too far-fetched. So much so that the theory didn't even stick with Aria's friends for long.
A-Team Material: Alison DiLaurentis
Although some flashback scenes show "A" tormenting Alison herself, it definitely seemed in the early seasons of the show that she might have been the ones ing her former friends. Even after Alison's body was discovered, the Liars were left wondering how someone knew so many things about them that only Alison knew.
Alison was "A" in the books and the "Alison" the Liars had known had actually been her twin sister, Courtney. The writers spent so much time insisting that "A" would be different in the show, so it makes sense to not have it be Alison. Still, it would have been a fun twist to have the girl who knew all the Liars' secrets be the one exposing them.
Total Red Herring: Lucas Gottesman
Lucas was a side character, primarily showing up in the earlier seasons of the series, and was known to be one of the many people at school that Alison had tormented. With Alison gone, Mona moved to take her place by mocking him and his friendship with Hanna. Though Lucas had motive to get revenge on the other Liars, the fact that Mona was on the A-Team and the fact that Hanna, someone he was so close with, was often hurt the worst by "A" makes it hard to take any of his more sinister moments seriously.
A-Team Material: Jenna Marshall
"The Jenna Thing" was introduced incredibly early in the series and was later revealed to be the night that Alison and the Liars accidentally blinded Jenna. After she returned to Rosewood, the Liars were less than kind to her and almost immediately suspected her to be the person behind the messages.
Considering some of Jenna's strange behavior and the fact that she had gotten the worst of Alison's torment when she was still in Rosewood, it would definitely make sense for Jenna to have been the leader of the A-Team.
Total Red Herring: Noel Kahn
Noel Kahn is a student at Rosewood High and was a member of the A-Team at one point in the series. Prior to this reveal, the Liars had him on their list of suspects for people that might be behind their messages, but their proof was never particularly substantial. Up until being revealed to be on the A-Team, Noel's bad behavior seemed more like teen rebellion than anything and never came across as truly malicious.
A-Team Material: Ezra Fitz
Ezra Fitz is a character that started off seeming like he was simply tangled up in some scandalous, romantic drama with Aria, but it became clear as the show unfolded that his past was much darker than it seemed. Because he had a history with Alison and the Liars were so often on the verge of finding out that he was Alison's "Board Shorts" beau, it would make sense for Ezra to be the one constantly threatening them.
Total Red Herring: Jason DiLaurentis
Alison's older brother, Jason DiLaurentis, was suspected of being "A" at various points in the series. Although he would have had motive, none of what the Liars suspected him to be "A" over was that concrete. The fact that Jason was easily one of the least suspect of the DiLaurentis family is definitely saying something, though.