Toughie. 2 is the only one who really seems like she's not being forced to smile at gunpoint, but she's not my cuppa. 1, 3, and 6 are pretty, but have smiles like I'm buying them from human traffickers. They're trying to get picked but just to make quota. (Unwillingness is a major turnoff for me.) 4 is probably best bet, overall. She doesn't look like a hostage and is still super cute.

Left, and for everything, not just the ass.