What is the optimum speed for takeoff flaps?
I assume the question means optimum climb speed.

From the Boeing 737 NG FCTM (3.30 Takeoff and Initial Climb):


  V2 + 20 is the optimum climb speed with takeoff flaps. It results in the maximum
  altitude gain in the shortest distance from takeoff. Acceleration to higher speeds
  reduces the altitude gain. If airspeed exceeds V2 + 20 during the initial climb, stop
  the acceleration but do not attempt to reduce airspeed to V2 + 20. Any speed
  between V2 + 15 and V2 + 25 knots results in approximately the same takeoff
  profile. Crosscheck indicated airspeed for proper initial climb speed.