Growing up in the South I've heard every version of southern accent imaginable (some are sexy, some are subdued, and some are just flat out redneck) so I don't notice it much anymore except in extreme cases of the latter. But some of of my friends and cousins from Colorado and Montana apparently loved how the girls talked whenever they come to visit.
My favorite north american accent on girls lately is the one that a lot of native Hawaiian girls apparently have. I do consulting work for a site design firm out there and pretty much their whole company is made up of girls. I don't know why it is (maybe I'm just picturing a beautiful, tan island girl on the other end) but I love talking to them.
My all-time favorite accent though is still an English or Austalian accent. Probably 90% of the girls I've met from the latter have all been smoking hot which helps that one out some.