Hydrazine has more hydrogen atoms per mL than water
Some simple calculations give the moles of hydrogen per mL in some possible alternative liquids. Taking into account density and molecular mass we get the following results:
Water 0.056 mol/mL 2 Hydrogens -> 0.11 mol H/mL
Hydrazine 0.0313 mol/mL 4 hydrogens -> 0.125 mol H/mL
symmetric dimethylhydrazine 0.0138 mol/mL 8 hydrogens -> 0.111 mol H/mL
methyl hydrazine 0.0191 mol/mL 6 hydrogens -> 0.115 mol H/mL
So, while some other liquid at STP hydrazine derivatives beat water, it looks like hydrazine has notably more hydrogens per unit volume. This is probably the record.