Is there a single "best" position for holding a fountain pen while writing?
You should develop your own style of writing. The most important aspect, is that the hold must be comfortable, especially if you use the pen for long periods of time.
Other aspects to consider, less important: esthetics, quality of the result (i.e. the writing), the coolness factor, the formal-vs-informal setup when you will use it etc.

I used fountain pens for more than 20 years. I use them less now, since the computer is quite incompatible with them. I speak from my own experience, as well as the experience of my family and colleagues.

I guess that holding the pen from the &quot;neck&quot; will allow you better control, e.g. fast writing. Holding it from farther away from the nib would allow you to fuel your imagination and create fanciful text / drawings.
The size of whatever you write / paint also depends on the position of the hold.