Best most efficient motor type to be used as a DC generator
Brushless DC motors (BLDC) or permanent-magnet synchronous motors (PMSM) are pretty much the same thing and are the best to use as generators. They generate AC, so they require a rectifier on the output. There is no way to regulate the generated voltage except to regulate the speed. You will need a charge controller or other electronic power converter to regulate the voltage to the extent that it needs to be regulated.

Commutator DC motors are heavier, subject to commutator and brush wear. With wound-field DC motors, voltage can be regulated by regulating the field current, but power conversion regulator could also be used.

Another alternative is an automotive alternator. That is a wound-field synchronous generator with slip-rings. Voltage is regulated by regulating the field current with an external regulator. Rectifiers may be built in or external. Newer alternators may be permanent-magnet synchronous generators with power converter voltage regulation.

Note that some BLDC motors have built in electronic controllers that must be removed to allow them to be used as generators.