Do you mean the real MC. Prob some Steel or Titanium Alloy along with several layers of Kevlar underneath. It would be the best metal as the UNSC basically would be funded by a government and you know governments, they would rather use the money than keep it in the economy.
the real MC has an inperiable high density alloy with a crystaline structure benethe. but its not the suit that makes MC. it his brains and detemination. if MC just waned to escape halo or the cov he would of used the index and blew himself to shreds. (Along with all biomass within 25 thousand lightyears.)