Fountain pen, or its cousin the quill? disposable ink-pack mechanical pen??
"Best pen" is entirely subjective. Having tried many, I came to prefer the cheapest pen known to man, the BIC Stic (medium point, blue), which I get for about 10 cents apiece in a pack of 20 at any store that carries school supplies.
I developed this strong preference many years B.C. (Before Computers) when I spent 6 months, 8 hours a day, writing data on file cards, and went through about 300 pens.
I happen to like a certain amount of tactile drag, yet not too much (fine points drag more to achieve the same ink on the page). The pens are easy to hold and have no protrusions that distract my fingers. Blue ink doesn't fade like black ink does. The results look consistent. If one misbehaves it can be stimulated by a few seconds over a flaming match. If it dies prematurely it's only ten cents lost, not a fortune.
Conversely I use an antique mechanical pencil that's at least 80 years old, for pretty much the same reasons (and it takes cheap bulk leads). If I had to buy another, it would be quite expensive as they're now collector items.