So, basically, anybody on this show could be A, right? Literally ANYBODY. Because that clue the waitress gives about "pretty eyes" could work for just about anyone.
Let's work through this week's suspects, shall we?
Exhibit A (ahem, pardon the pun): Alison herself!
She may or may not have been a hallucination that appeared to Emily after the incident with the car and the gas in the garage. She was wearing a pretty un-Alison sort of outfit, with boots, jeans and a pretty tank. Usually she's all Sookie'd out in sundresses and things, but this outfit is fairly practical.
Does that mean Emily was dreaming of Alison dressing like her, or does it mean that Alison has started wearing pants? Who knows?

Eyes: Pretty. Other clues? Has name beginning with the letter A. Possible obstacle to being A? She is dead. (... OR IS SHE?)
Exhibit B: Maya!
The world's oldest teenager reappeared again this week, to tell Emily that she wants to just be friends for the time being. I don't know how convincing that argument is when you're wearing a see-through shirt with a visible black bra.

Eyes: Pretty. Other clues? Was she really off at Bible camp this whole time, or was she lurking around being A? Possible obstacle to being A: she has real feelings for Emily, and I can't imagine her wrecking Emily's swim team career with steroids.
Exhibit C: Jackie!
Ezra's ex is behaving like a crazy person, trying to get him back by whatever means necessary. And she thought nobody would notice her plagiarized French paper? Has she met Detective Spencer Hastings?
This meh purple top and meh jeans really are no competition for Aria's wackadoo wardrobe. If you're trying to steal a guy from a girl with 10-tonne earrings, I think you need to step it up in the style department.

Eyes? Meh. Other clues? Meh. Obstacles to being A? She'd be messing with Aria way more if she was A. Aria's really come out unscathed, for the most part, by A's schemes.
Exhibit D: Jason!
Alison's (and maybe Spencer's?) hottie ex-underwear model debuted a shorter 'do this week that made him look kind of like he wandered in from the set of a 2004 episode of One Tree Hill.
I mean, he looks cute, but less cute. Somehow. I think he may need to go shorter, or something. Or take off his shirt...yeah...he needs to just take off his shirt.

Eyes: Pretty-ish. Other clues: General stalkerishness, mysterious goings-on in his house. Obstacles to being A: if he was A, he could force Aria to break up with Ezra to be with him. Which hasn't happened. Also, he seems kinda nice.
Exhibit E: Ezra!
Aria's boyfriend/ex-teacher may seem squeaky clean, but he did show up in Rosewood at the same time that A started stalking the girls. He used to wear suits and vests as a high school teacher, but now is a jeans-wearing college professor.

Looks more like a college student than a professor, but whatevs. Eyes? Pretty. Other clues? See above re: appeared in Rosewood at the same time that A began creeping. Aria gets less hassles than the other PLLs, indicating A may like her best. Obstacles to being A? The show's writers would be massacred by angry Ezria fans if it turns out their entire relationship was a lie.
Exhibit F: Toby
Spencer's (ex-)BF casually mentioned that his brake lines were cut, as if that wasn't OBVIOUSLY the work of somebody out to get him. And then he weirdly starts wondering, out of nowhere, what their babies would look like? I'm a fan of Toby's usually, but he was weirdly creepy this week.
And on top of that, he wore this out-of-character and wildly unattractive purple button-down shirt to visit Spencer in jail.

Eyes: Pretty. Other clues: Weird behaviour this week. Befriended first Emily, then Spencer, indicating a need to get close to the PLLs one way or another. May blame them for sending him to jail over The Jenna Thing. Obstacles to being A: Wants to rebuild his reputation after being accused of killing Ali, so why would he mess that up through psychological warfare with the PLLs?
Exhibit G: Garrett and Jenna.
This is an obvious choice, after their scene this week where they seemed to be confessing to something... but what? Killing Ali? Placing the anonymous call that trapped the PLLs with the shovel? These two are either A, working for A, working with A, Ali's killers, or just two deeply creepy people.
And Garrett, for the first time, looked kind of hot to me this week. Maybe it's because we now know he's truly, truly evil and only became a cop for nefarious reasons?

And I'd like to mention because I haven't yet, that Garrett used to be on a Canadian/Australian sci-fi teen soap called Guinevere Jones playing the reincarnation of Lancelot. And I still harbor some resentment towards him for stealing the reincarnation of Guinevere from the reincarnation of Arthur. Basically, I always knew he was up to no good, so this week's reveal wasn't that big a surprise.
Oh, and Jenna still has the prettiest hair of any blind girl on TV. And a fabulous sense of style for someone who can't see what she's putting on in the morning. Does Toby help her pick out her outfits? Or are they somehow done up with Braille?

Eyes: Garrett - pretty. Jenna - covered in sunglasses and scarred (or are they??). Other clues? EVERYTHING. Obstacles to being A: they seem like an obvious choice now, and the show wouldn't be that obvious. (...OR WOULD IT?)
Wildcards:
Any of these tertiary characters could be A, but that would just be weird.
Officer McCreepy!

He's baaack! Remember him? He's the police officer who was obsessed with proving the PLLs killed Ali, and then started sleeping with Hanna's Mom until he disappeared off the show for basically an entire season. Remember him? I only do because I recapped the pilot a few months ago.
Ashley is like, "Ugh, I had forgotten about him too. Please don't let us have any scenes together."

Dr. Wren!

He has pretty eyes! He behaves inappropriately and has reasons for wanting Spencer to break up with Toby. Plus, his British accent could be construed as being somewhat evil, in a James Bond villain sort of way.
Isabelle!

OK, there's basically no way that she's A, but I wanted to include this picture of her in her wedding dress that Hanna puked on last week. Yay for dry-cleaning! [I thought Tom (maybe? or someone else) said that they had cleaned Isabelle's dress...but this isnt the same one from last week...plot continuity mistake? --Ed. Kate]
Caleb!

He has pretty eyes (and a pretty everything else) but I'm pretty sure he isn't A for the same reasons that I've excluded Ezra and Toby. The fans would FREAK! OUT! if any of these boys were shown to be A. And the psychological manipulation A uses seems like it would come from a girl, somehow.
And Caleb got his hair cut this week too, I think. Like Jason, it wasn't a good decision and I hope he takes lots of vitamins to grow it back out to mid-90s Jordan Catalano lengths ASAP. And then takes off his shirt. Call me!
And I will see you bitches for the Halloween special, which CANNOT COME SOON ENOUGH! - A