What is the substance with the highest specific heat?
This may not be the absolute highest, but on a mass basis hydrogen gas has more than three times the specific heat as water under normal laboratory conditions.  Diatomic gases under ambient conditions generally have a molar specific heat of about $7\text {cal/(mol K)}$, and one mole of hydrogen has only $2\text {g}$ mass.  Thus $3.5\text {cal/(g K)}$ for hydrogen versus $1\text {cal/(g K)}$ for water.

Helium, contrary to the question, is $5\text {cal/(mol K)}$, not $7\text {cal/(g K)}$.