The rides out of Salisbury are nice and flat and make a good first century unless its windy.

The BBC Civil War Century is a good runner up as it has a bail out at ~60 miles that's all down hill to the start. But if you do go for the full century it is hilly!

CBAR: http://www.mrsnv.com/evt/home.jsp?id=2372
CWC: http://www.baltobikeclub.org/index.pl/cwc
SGC: http://www.seagullcentury.org/