"Which oxidizing agent is more powerful" seems to depend on what is getting oxidized.
Consider for example oxygen vs chlorine. It is my understanding that chlorine is the more powerful agent for oxidizing sodium metal. The enthalpy of formation of sodium chloride is almost twice as negative as the oxide on a per-sodium atom basis (about -400 vs -200 kJ/mol Na). However, if we are oxidizing hydrogen the picture reverses. Assuming everything stays in the gas phase and below about 600C at atmospheric pressure (see here) the equilibrium favors H2O rather than HCl.
It seems that when we are talking about dilute aqueous solutions we can define oxidant power based on standard electrode potentials, but when we generalize this concept (even if we standardize temperature and concentrations/partial pressures) there is no "right" answer unless we define a standard set of reducing agents to compare to. Is this correct?