Which airfoil would be optimum for a glider?
For a particular task of optimizing an airplane design for a specific point, you will need to consider the following characteristics, together with the endurance parameter:


The pitching moment of the airfoil, (which will define the tail design, hence will affect weight and CG range and controllability)
Tolerance to contamination and geometric impurities (especially CFD based methods may converge to non-robust shapes)
Stall behavior (does it stall mildly or violently, as a wing)
Wing weight (spar location, spar web thickness)
Aeroelasticity (wing structural modes, static and dynamic aeroelastic properties)
Manufacturability (if somebody will really build it!)


Many of these would require working on a conceptual or preliminary sizing task, where you would design a planform, twist, dihedral, tail volume, CG, mass estimation, aeroelasticity computations etc.

Number 1-2-3 could be more vital and would help pick an airfoil with less efforts than the others.