What material conducts electric current better than an insulator by not as well as a conductor?

"insulator" and "conductor" are really the same just that electric conductivity is many orders of magnitude higher in a conductor than in an insulator. So if you have several kilometers of silver metal it will insulate (electrically) as well as a few hundredths of a millimeter of Teflon.

Semi conductors lie between conductors and insulators with the added advantage of being able to change conductivity if they are configured correctly and if control voltages are placed across them properly.