When making a manga, there is a very important step, the inking!
For this, most mangakas use feathers with different characteristics and uses, here they are:
The G-Pen:
The manga specialist and not without reasons! It is widely used for inking shōnen.
The G-Pen is very flexible and offers large variations in lines depending on the pressure and inclination of the nib. This will make your boards very dynamic!

The Maru-Pen
This feather is much smaller and allows you to ink details, hair, etc. It is most often found in Shojo style manga. It is quite difficult to handle.
Its small size allows very fine and fluid lines, it is still possible to vary the thickness according to the pressure but much less than with the G-pen.
It is also much more fragile and wears out faster!
La Saji-Pen / Kabura-Pen
The Saji-Pen is a much stiffer nib, allowing for a more uniform stroke.
Because of its rigidity, it does not allow to make large variations of lines like the two previous feathers. It is often used by beginners, for the realization of backgrounds, trees etc. Its handling is very simple!
Conclusion
There is no "best pen", it depends on how you are going to use it!
Indeed, one can use a maru-pen for an action manga and a g-pen for romance, slice of life etc. ( Kōhei Horikoshi , creator and designer of the manga My Hero Academia , use a Maru-Pen to ink! Watch the video )
In addition, the Saji-Pen is categorized as a "beginner" feather while for example the famous mangaka Naoki Urasawa ( 20th Century Boys .. .) uses a Saji-Pen to ink his boards!
The best is to experiment and draw with passion!
If you want to get these feathers quickly and cheaply, it's here !