Most endothermic room-temperature reaction
Don't know if these are quite in the spirit of what you are looking for since, with the exception of the first reaction, the entropy change alone is not sufficient to drive them  (i.e., at room temp., their equilibria lie far to the left) (and, for ozone formation, the entropy change is actually unfavorable).  Furthermore, ozone formation and photosynthesis aren't single reactions, but multi-step processes.  But they all do take place at room temperature (or below):

Sublimation of ice (yes, snow can evaporate!):  2.8 kJ/g

Formation of ozone (O$_3$) from O$_2$ (photochemical, occurs in stratosphere): 3.0 kJ/g

Photosynthesis: 7.5 kJ/g

Electrolysis of water: 15.9 kJ/g

The values I've listed are all enthalpy changes (at constant pressure, the heat flow is equal to the change in enthalpy; at constant volume, the heat flow equals the change in energy).  In addition, by convention, the sign of the enthlapy for endothermic reactions is positive, since heat flows into the system, so the enthalpy is increased (not negative, as you've used).