The liars at their prettiest?
Aria - Pilot. I was completely mesmerized by her looks. I think she looks best with long hair and that right amount of eyebrows (her eyebrows are too thick right now).
Hanna - Season 3. The short hair was pretty great.
Spencer - Father Knows Best (02x02) where she had her hair just flat-out straight (https://heftyhannahscloset.files.wordpress.com/2014/08/image821.jpg). She looked stunning there. By the way, she also looked the best after the time jump. The bangs work. Oh, and also the 40s noir films homage! Her face is made for that era.
Emily - Emily looks constantly beautiful. It must feel nice being her.
Alison - Season 3. She's grown out of that tween face.
Other characters worth mentioning:
Caleb - Season 2. I like the long hair and greasy look better.
Toby - Season 6.
Ezra - Meh, never thought he's very attractive, to begin with.
Jason - Season 2 when we first saw him.