Cutest unit? Easily non-awakened Sandy, or more specifically her salamander.

If it's a more general question of best art, then it's a toss up between Mikoto, Nataku, Koharu, and Lien (the first three of which share an artist).

Also worth mention is Ashera, as while the girl herself isn't particularly notable, the difference in style of the arrows compared to the rest of the images lends a rather appealing ephemeral quality to them, making them look magical in a way.

Looking less at the art style and more the design of the girls, there's Rino, who has a fun kind of magic detective look going on.

There's at least a few others, though I can't recall them off hand given how many units are out on DMM. Similarly couldn't say who my favourites are as a result. It likely varies some with my mood to be honest.