Second the motion on Spencer Tracy (up to 1950). Jimmy Stewart, John Wayne, James Cagney, Edward G. Robinson, Barbara Stanwyck, Alan Ladd. These are all "big stars" - I need to think more about some of the high-quality but lesser known actors, especially the character people. (Adding one of those right now, I'll say Frank Faylen. He always seemed to really be whatever character he was playing.)