Obtaining constant mechanical energy from a DC Machine
Among the commutator DC motor types, the series motor provides the widest speed range for constant power operation. However a separately excited motor can be controlled with a combination of armature and field voltage control that might have some advantages. Which configuration is better might be determined by your definition of best. You might need to look at what is available for the specific torque and speed requirements of the application.

The use of commutator DC motors has been declining in favor of other alternatives for about fifty years. They are an interesting machine to study and students can probably learn a lot from the experience, but I don't think they are considered very seriously for use today.

I don't have enough interest in commutator motors to open a book and try to sort out the parallel paths question.