Green Dot would give you the best Lift/Drag ratio, hence the best angle of climb (or holding speed).
For want of anything better and for practical purposes I use Econ Climb Speed at Cost Index 0. This was Boeing logic (I know, Boeing) but laws of physics are universal and the FMC/FMGC are the same make.
I have read somewhere in one of the Airbus FCTM versions that you can use Turbulence penetration speed to get the best rate.
And another consideration would be whether you refer to a static (constant speed) vs a dynamic maneouver (such as on an RA) where you'd get the best rate by trading speed energy for climb rate.
My two cents.
