A dime is a perfect ten (i.e. a girl that is extremely attractive; in the top 10% of all women). If you take all the dimes in the world, and then rate them from one to ten, the tens among that group are "dimes among dimes" (the top ten percent of the top ten percent).
A dime among dimes is a girl that is in the top 1% of girls worldwide. She is impeccably beautiful. A dime among dimes makes other dimes look ugly.
Put simply, these girls are smoking hot.
Holy crap... Look at that girl!
Perfect legs and ass. Great breasts. Beautiful face and hair.