My vote is for British born actress Jane Seymour.
I can't say I buy the idea that Americans find all British accents appealing. An upperclass-RP accent is fine, but lower class British accents aren't exactly easy on the ear - and I'd bet most Americans find them incomprehensible.
Americans show excellent taste in not falling for Stephen Fry, though. Here's hoping his fame remains confined to the other side of the Atlantic.