"Attractive" in this case, meaning closest to attractive traits for human women, since that's the easiest comparison to make and standard to judge by? As far as when it comes to feminine rock aliens. Lapis has got to be up there, since her size is comparable to a human, in fact she probably has the closest body to human of any gems. Compare with a gem in the same ballpark, Pearl, but Lapis isn't creepily stick-thin. She's got kind of just an "average" woman's body for humans in the setting, minus distinct sexual features like booba. If she wasn't blue, she'd probably be the hardest to notice of all gems, among a crowd of humans. Basically when the only drawback for passing as human is color, Lapis beats out against pretty much all other gems who have worse or more numerous negatives for passing as human.
Contenders for the most attractive gem (by human standards) would be Rose quartzes since they seemingly are mistakably human women, just obviously much larger.
After that... Blue Diamond? She seems to have the most womanly humanoid body among gems. Again though, ignoring size.
It's worth noting that even gems with fucked up size/proportions shouldn't be discounted, because many humans in the setting also have fucked up proportions. So like you couldn't write off a Ruby as not being attractive, when you've got like Sadie who is the same size/proportions as a Ruby.