The Spencer Tiara that Princess Diana wore on her wedding day in 1981 might be a top contender. Given Prince Harry and William's devotion to their late mother, wearing the same tiara would be a beautiful gesture for Megan to make on her big day.
In typical royal jewelry fashion, this tiara actually comprises other pieces. The middle part of the tiara was originally a wedding present given to Lady Cynthia Hamilton (Diana's grandmother) in 1919 on her big day. That same year, the Spencer tiara got an upgrade with a topper piece, then had one more piece added before the 1920s. It underwent one more revamp in the 1930s and has remained the same ever since.
Diana's sisters, Sarah and Jane, as well as Victoria Lockwood, the first wife of Charles Spencer (Diana's brother), have all worn the tiara.