Most dense hydrogen containing composition
A few binary metal hydride candidates:

Magnesium hydride gives about 0.11 grams of hydrogen per cubic centimeter, and unlike water it can release its hydrogen leaving the element behibd on simple heating.  It does, however, require 287°C to decompose, limiting reversibility.

Lithium hydride offers nearly the same hydrogen density as the magnesium compound but decomposes at a much higher temperature, limiting its utility.

Beryllium hydride approaches approaching 0.12 grams hydrogen per cubic centimeter.  But it has several drawbacks; beryllium is expensive, it's a serious health hazard and the element does not react directly with hydrogen, making an element/hydride cycle impossible.

Aluminum hydride, with a favorable stoichiometry, comes inat nearly 0.15 grams hydrogen per cubic centimeter and also releases its hydrogen at 150°C.  Like magnesium hydride it has been considered for hydrogen storage.  However, reversible cycling between the element and the hydride is again inhibited because aluminum and hydrogen do not react directly except at very high pressure (10GPa for aluminum hydride versus 20 MPa for a direct synthesis of magnesium hydride).  As with beryllium hydride, getting the aluminum back to hydride form requires indirect methods with other hydride sources.

A ternary compound with 0.15 grams of hydrogen per cubic centimeter is $\ce{Mg2FeH6}$, which can be made from the elements at 2-12 MPa pressure after compressing the metals into pellets.  This compound can also release its hydrogen thermally and is one of many ternary hydride candidates for hydrigen storage.