Aluminum (Or if you're RAGE/571, wood
)
Don't use steel. Ever. We did in 2002, for some insane reason thinking we would need the extra strength with all the robot on robot contact. Well, a frame made of 1 1/4" steel angle 0.125" thick ended up taking almost half of our allotted weight--with nothing on it.
Needless to say, aluminum would have worked just fine, and we would have had the weight for a lot of components that got scrapped because the thing was so darn heavy.
P.S. Perhaps you want to make this a poll.