Idle reverse is when the reverse thrust mechanism has been engaged but with the engine on idle. You won't hear the loud noise usually associated with reverse thrust. Pilots use this setting when, at touchdown, there is sufficient runway ahead not to warrant full reverse thrust, which adds wear and tear on the engines. The plane will slow down a little, usually enough for the wheel brakes to finish the job of decelerating.
As for strongest reverse thrust, I would bet on any of the three engines that power the 777.
The aeroplane has unveiled for us the true face of the earth. -Antoine de Saint-Exupery