  We present an inelastic neutron scattering study of the structurally simple
single-layer compound HgBa$_2$CuO$_{4+\delta}$ close to optimal doping ($T_c
\approx 96$ K). A well-defined antiferromagnetic resonance with energy
$\omega_r = 56$ meV ($\approx 6.8 k_BT_c$) is observed below the
superconducting transition temperature $T_c$. The resonance mode is
energy-resolution limited and exhibits an intrinsic momentum width of about
$0.2 \mathrm{\mathring{A}^{-1}}$, consistent with prior work on several other
cuprates. However, the unusually large value of the mode energy implies a
non-universal relationship between $\omega_r$ and $T_c$ across different
families of cuprates.
