There is no winning in this situation.
Everyone is right. The girls are fishing for compliments. Someone else above said that it is either because they just want more compliments, or they really don't believe that they are beautiful (low-self-esteem) and are still fishing for more compliments.
Problem is...if you compliment more...it will be an endless loop, or they just will write you off as an AFC or as a flatterer. If you say something like "Well, you can't take a compliment" if they're self-esteem really is that low, they will really retract from you.
Maybe....and this is a maybe, it would work to say something like,
"Well, it's a matter of opinion, and mine's the only opinion that matters!" (cocky/funny?)
or
"Well, you can believe me or not, but I know what I think." and then continue the conversation.
Personally, when someone calls me "hot" or "beautiful" or any compliment, I usually just respond with..."I know."

(there are reasons behind this)
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Hobbit says that I should tell you I'm female.....so.....
HI!!! I'm a Girl!
