The most efficient option is natural ventilation, typically via windows that can be opened.
In general, individual fans without ducting are likely to be the most efficient of the mechanical ventilation options. Ducted individual fans can also perform well, as long as the intake, ducting and discharge are reasonably sized. Although not a requirement, fans should have automatic dampers or shutters so that air does not leak through them when they are not operating.
A fan with some form of operation control such as manual on/off or interlocking to the light switch will be considerably more efficient than a continuously operating fan.
Additional options for multi-dwelling developments
For multi-dwelling developments, the individual fan options are likely to be the most efficient of the mechanical ventilation options. Even though the small fans themselves have poor efficiency, this is offset by their short operating time.
Of the central systems, the most efficient is an individual motorised damper into a central duct with an efficient control (e.g. interlocked to light); with the central fan having a variable speed drive. Even though the central fan operates continuously, it has a higher efficiency than the small fans, and the VSD allows the energy to be reduced considerably in periods when few of the connected apartments are operating.
The central ducted system operating continuously scores very poorly in BASIX because it exhausts from all apartments all of the time, so has a constant high energy input.