I guess you mean under ordinary conditions, at room temperature. It turns out that how well a metal conducts electricity is an excellent guide to how well it conducts heat. I think that pure silver is the best conducting metal at room temperature (or at least nearly so). You may have trouble finding a good silver wire, but copper works almost as well.
As for the worst one, remember that there are all sorts of metals made out of mixtures of different elements. If you had one that was a really bad conductor, somebody else could make another one that was just as bad. Of the common ones youíll find in use around your house, stainless steel is one of the poorer heat conductors.
To show the difference between two metals, you might take two thick wires, with the same thickness and length, of the different metals. Put one end of each in a big pot of very hot water, and put the other end of each in its own cup with an ice cube and a little water. (Donít use water so hot that you could get scalded.) Try to make the cups as much alike as possible, and use ice cubes the same size. Then see which ice cube melts faster. Thatís the one that was getting heat delivered to it faster. (I havenít tried exactly this experiment, but have done a similar one.)
By the way, Iím sure we have some other answers explaining a little about WHY different metals conduct heat differently, so you might want to use our "SEARCH" button too.
(published on 10/22/2007)