What is another word for a paper that shows legal ownership of property?

It depends on whether you are talking about personal property or real property.

A deed is the written instrument by which ownership of real property is transferred in recorded land systems. In land registration and Torrens sytems, once a deed is registered, a Certificate of Title is issued in the name of the owner.

With personal property, a Certificate of Title is the usual document that shows the ownership of the property, generally a motor vehicle.