I figured Christendom's holiest relic would be the Grail or Spear of Longinus.
The Ark of the Covenant would be more important to Hebrews.
Since the Ark of the Covenant was before the incarnation of Christ, it would be about the holiest relic to both Jews and Christians...seeing as how it hosted the presence of the same holy God....
Of course for Christians the communion chalice or a piece of the cross would rival it...however the Ark of the Covenant is NOT strictly a Jewish relic.