Time for an update, now that Rosemarie DeWitt can be crossed off my list (though I'd still love to get a better look at hers at some point):
1. Saoirse Ronan
2. Michelle Dockery
3. Mia Wasikowska
4. Emily Deschanel
5. Miranda Lambert
6. Anna Kendrick
7. Lauren Ambrose
8. Ellie Kemper
9. Bella Heathcote
10. Sara Bereilles
Incidentally, there are also some notable actresses whose bellybuttons I've only ever seen via nudity (off the top of my head: Bryce Dallas Howard, Emily Mortimer, Emily Watson, Kat Dennings, Laura Linney, Romola Garai, etc.), which does us no good here. :-\