Assuming you mean the highest efficiency at low loads in Buck DC-DC regulator to provide 5V, I found a Rohm design with 50% efficiency at 0.1% of Imax which indicates the overhead current was 1/2 of the load but then around 90% at 1~3A.
So your load duty cycle determines the overall energy efficiency.
In order to be a stable control system feedback needs to be in the same units as the regulating variable. Such as regulating forward current using current sense and forward voltage with voltage sense and not the integral of voltage for current from reactive loads to avoid loss of phase margin and efficiency losses during switching so it must also use zero current switching near zero voltage phase which gets very complex in design terms with traces being 10nH/cm and stray capacitance. So low load efficiency is a tradeoff with many other variables.
The best design must sacrifice or tradeoff many variables for power efficiency, dynamic load range, input/output ratio limits, step load margin, max ripple etc.
Once you have a spec, then all parameters in a design can start or you can examine suppliers to these specs. But you cannot examine 1 parameter like **low load current efficiency "in isolation."
But if you mean, lowest quiescent LDO for 5V with some Imax... this is a different topology that is possible with CMOS LDO's. For example 1uA idle with 1A out max is possible.
Or you simply use a PMOS switch with >1M off resistance and say 10 mOhm on resistance for load regulation error RL/(RL+Ron)*100%.
Considering an AC-DC converter with a high-efficiency requirement that draws 1W is challenging (due to cost/W sensitivity) and ensure the safety to disturbances to humans without compromise.
The best solutions recentlyEU NA found are there.
3W continuous @5V 0.6A
< 0.075 W no load power consumption
Insulation 4242 Vdc 1 minute @ 10mA ( from Ic=C dV/dt )
safety approvals GS (IEC/EN 60950-1, IEC/EN 62368-1)
EMI/EMC CE (EN 55032, CISPR 32, EN 61000-3-2, EN
61000-3-3, EN 55024, CISPR 24)
Climate 20~40'C max ambient 85% RH max
OCP = 1A auto-recover