Is there a single "best" position for holding a fountain pen while writing? Certain positions are taught in school for holding a pen (not typically a fountain pen or quill in modern times). A variety of grips/positions can be seen in online photos. For example, from Amazon:
 
These two pictures show very different positions of holding a fountain pen.
Different grips will provide different control and comfort. This question focuses on just those two objectives. Ignore, for example, specialized grips that left-handed writers sometimes adapt with fountain pens to keep their hands out of the wet ink.
This question also focuses on just fountain pens, since pressure requirements are different from, say, ballpoint pens, pressure affects the ink line, and writing on fountain-pen-friendly paper often provides less feedback than most other types of pens.
Using joints and muscles in a natural way must also play a role. For example, one grip may not be familiar to the writer, but could be learned and is just as easy on the body as a more familiar grip. Another grip might make your hand tired or painful more quickly.
In the absence of injury, disease (like arthritis), or other abnormalities, human hands work pretty much the same (ignoring differences in hand size, finger length, and the like), so it seems logical that certain positions that optimize control and comfort should apply broadly to most writers engaged in normal penmanship (excluding specialized writing like calligraphy), and writing characters in the typical range of handwriting size.

Is there a grip/position for writing with a fountain pen that optimizes control and comfort, and is recognized as being superior to other positions for most users?

A poor position (e.g. comfortable but reduced control, or that leads to pain after prolonged use), may not be immediately obvious as poor. Low quality writing could be attributed to insufficient practice by a new user, or eventual pain might not be attributed to inadequacies of the grip. A new fountain pen user has no basis for comparison. If it is the case that different positions work better for different people, how do I judge whether the position I'm using is less than optimal?
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.