Most powerful oxygen free radical is:
Most powerful oxygen free radical is Hydroxyradical:

Least powerful Reactive Oxygen species is Hydrogen peroxide.
Precursor of all reactive Oxygen species is Superoxide radical.
H2O2 is not a free radical but a ROS.

Reference- Review of biochemistry by Rebecca James Perumcheril Pg-411
Hydroxyradical
Hydrogen peroxide
Superoxide radical
Both 1 & 2