Magna Carta and the companion Carta de Foresta or Charter of the Forest to Canada.
The four original charters dating to 1215 never leave England but a copy from 1225 might be allowed to make a four city tour of Canada.
Magna Carta is considered to have set the west on the road to democracy by providing that "no one is above the law," a body blow to the divine right of kings. Carta Foresta which was enacted a few years later set out a bill of rights for freemen that, again, restricted the control of the monarch. Some parts of Carta Foresta remained in force in Britain into the 1970s.
Bringing these treasures to Canada would be invaluable to the country and our Parliament where the current monarch seems to have forgotten the bit about "no one is above the law." There does seem to be some concern that sudden, simultaneous exposure to both Charters could cause Harper to spontaneously combust.