Well, Tmk we haven't actually seen what Yamamoto Can Do so this is really a speculation battle, But My money is on Hiei. In general yyh characters are > then there bleach counterparts.
" With all honesty the strongest Bleach characters are only barely on the level of A class in terms of Yu Yu Hakusho. And for an A class fighter even being in the hundred meters radius of an S class fight means they're dead in the next second...simply from the bakcslashes of the clashing two powers. And Hei IS an S class Demon afterall:) "the strongest bleach characters (captains) may be about A class level but Yamamoto is more than twice as strong as a normal captain. Also Kuwabara by the end was supposedly A class and I can't see him beating any of the captains.