There are many different mediums that can be used from traditional to digital media, and possibility is endless. Bruce Nauman works in wide varieties of medium, such as sculpture, video, film, printmaking, performance, and installation, etc. Richard Serra works with enormous amount of steel.
I like artists that choose materials according to the concepts they are working on. I like the fact Mark Dion puts focuses on materials, and he has used a log he found in a forest to carry his message. Materials have on their own meanings before artists even touch them, and in his case, the collaboration of the meaning of the material and his own concept matched perfectly.