Well, Vibranium would probably be the Gold Standard.
I'm thinking more down the lines of aluminum tubes, I-beams, etc. Even something like building a jig to turn sheets of aluminum (such as from a roll of flashing) into fan-fold/accordion components with a greater strength-to-weight ratio.
I also would love to hear any ideas for affordable composite construction. I'm not sure of the real-world practicality (or environmental suitability), but I was toying with the idea of using spray insulation on wire armatures with strategically-placed "hard-points" in prototyping different configurations.