It really depends on the situation.
If ATC is hoping to climb you over conflicting traffic or terrain, or before reaching a waypoint - at some specific place, then GD (best angle or gradient speed = Vx) will get you higher in the minimum distance (because you're flying slower = more time available to climb before reaching the crossing point).
To reach the new altitude in the minimum time (but not the minimum distance) then turbulence speed (generally) is your best climb rate (Vy).