What Is the Most Efficient Electrical Generator

There are a number of ways to do this and none is 'best' as all are compromises, and the final configuration depends on what assumptions are made.

 However, if you wanted the industrial norm of constant voltage constant frequency AC then you almost certainly need to store energy from the bikes and produce the AC from the energy store. As advised below, the most likely bike power producer would be a permanent magnet alternator producing multiple phase AC (usually 3 phase). Voltage and frequency and power level are immensely user dependant and the best method is likely to be to convert this output to DC, store it in a battery and then produce fixed voltage fixed frequency AC using a DC to AC converter - an off the shelf product.

A good way to handle the bike AC is to arrange for the generator alternator AC Voltage to be higher than the battery DC voltage at all useful power output and speed ranges, convert the AC to DC and then "buck convert"(= voltage down convert) the DC to battery level voltage. A charger-controller would handle the input from all bikes and manage battery charging. Depending on design requirements users may be requested to pedal at constant power or constant voltage (both of which can be enforced by a controller with feedback to the user) or be free to provide input as desired.

It would be possible to transfer energy directly from bike rectified DC via down converters to the DC to AC converter input directly without battery storage - and this is essentially what happens to most of the energy when bike user input is <= load, but completely batteryless operation would be difficult as the battery provides a stabilising influence and, in a properly designed system, an energy source that has no drop puts to below load requirements.