not a locomotive as such but a self propelled passenger vehicule, and that would have to be a narrow gauge version of the perry people mover.
second would be pretty much all narrow guage multiple unit equipment, reguardless of power source.
especially including the old japanese standard which is narrow gauge by north american standards, and especially among them the kiha 80 series and their later derivatives. although there's a narrower gauge system, even for japanese standards that i forget the name of, and don't much care for either its erlier nor later color scheme, but the equipment is really, or looks like in the pictures, which is all i know of it, just right.
then there's my own design, of a 30in guage local transit m.u. vehicule.
something i see as some day in some places replacing most of the functionality of the private automobile when those become no longer practical, which, if not in my lifetime, then quite likely in someone's not too far past it.
as for favorite types of propulsion, if i haven't already said so in this thread before, which at i guess i might have, any sort of stored or clean energy propulsion. flywheels, compressed air, batteries, fuel cells, cable car with the cable wound by wind or air or whathave you, essentially anything that doesn't involve burning anything at any point.
cat and 3rd rail are ok too, but onboard power, even if its stored energy, does have certain advantages.