Which is the best moderator?

Protons are the best at slowing down neutrons, however they can also absorb neutrons (and form deuterons) in competition with the fuel. Heavy water or D₂O slows down neutrons but does not readily absorb thermal neutrons, there it is considered a better moderator for thermal fission reactors.

Moderating ratio is a figure of merit for moderators.
http://www.tpub.com/content/doe/h1019v1/css/h1019v1_131.htm

 

But that wouldn't work for a hot reactor. D2 doesn't compress to a liquid but for pressures well beyond what we normally achieve terrestrially.

 

Supercritical fluid would seem more appropriate. The moderator needs a certain density.

One way to determine the superiority of deuterium to heavy water would be to determine the density of deuterium necessary to give the same moderator ratio as heavy water. From that density, then determine the pressure required. Then determine the necessary requirements of the pressure vessel and its material to contain that pressure.

 

Since you brought up the idea of solid hydrides, which is being considered in some high temperature concepts, e.g., SCWR, the other complication of high pressure deuterium would be the propensity for the hydrogen to diffuse into the structural metals and forming hydrides. This would be bad news for steels or Inconels as it would undermine their structural integrity, especially when it comes to shutdowns. Probably the primary system would have to be cladding in Al or Cu or some other element that does not readily allow diffusion of hydrogen into the metal or formation of hydrides. However at the pressure required to maintain a supercritical deuterium fluid, I'm not sure there is a metal that could handle it.

Bare in mind, a moderated reactor has to be cooled, and there is only so much volume for the coolant, moderator, fuel and structural materials.

Certainly one could design an epithermal or fast reactor spectrum, but then there are issues of control and stability - depending on various design factors. Nevertheless, the OP was concerned about 'the best moderator'.