what's the best beginner fountain pen?
A comparison between the Pilot Metropolitan, Pilot Kakuno, Lamy Safari, and Kaweco Sport
tl;dr
Fountain pens are highly subjective, so I recommend trying these pens however you can (at a store, borrowing from a friend, etc). That being said, in my experience the best beginner fountain pen is the Lamy Safari. If you're really not sure if you'll like fountain pens and/or want to save some money, go for the Pilot Kakuno, which is a solid choice that I enjoy using just as much as the Safari. If you want a pocket pen for on the go use, get the Kaweco Sport. And I would only opt for the Pilot Metropolitan if you're sure you want a hefty metal pen.
For German nibs (the Lamy and Kaweco), I recommend going with an extra fine (EF) nib if you write small or want a thinner line and a Fine (F) if you write larger or want a thicker, wetter line. A medium (M) will get you an even thicker line, one that will probably bleed on most papers.
Nibs from Japanese manufacturers like Pilot run about a size smaller than their Western counterparts, so I recommend Fine or Medium nibs for the Kakuno and the Metropolitan. I have a medium and it doesn't feel wide to me at all. The Fine (F) nib will lay a very thin line and produce a lot of feedback (i.e. you will feel more friction between the nib and the page).
My story with these pens
I recently bought a beautiful Kaweco Skyline Sport, which is the last of (from what I can tell) the four most popular beginner fountain pens. Some might include the TWSBI Eco in this list; while I agree that it is a fantastic starter pen, it's slightly more expensive and less beginner-friendly (since it isn't compatible with cartridge ink) than these four.
My first pen was the Pilot Metropolitan in black with a Fine nib. It was a good introduction to fountain pens since it wrote well out of the box. But I found that the metal bodied pen was ultimately too heavy and unwieldy for me. It doesn't post well and feels just too short when unposted. I didn't like the short grip that got slippery when my hands sweated and just how hefty the pen felt in my hand. I've since given this pen away. The value offering is great from Pilot, but for those aforementioned reasons I never found myself itching to use it after I got other pens.
Since I liked the Pilot nib on the Metropolitan but not the body, I got a Pilot Kakuno for my next pen, which solved pretty much all the problems I had with the Metro. Even though it's cheaper than the Metro I liked it much more—it has a subtly triangular grip that coaxes your fingers into the right positions (and it's far less coercive than the grip on the Lamy Safari). The plastic barrel doesn't roll, and I enjoyed writing with it posted. My only concern with it are the two small holes in the cap, which I believe are designed to reduce the risk of children choking on the cap. This sounds like a good idea, but the risk of it leaking is enough to deter me from taking it with me on my travels.
I used the Kakuno for a few months as my daily driver until I received a Lamy Safari (Fine nib) for my birthday last year. I know the Safari can be a bit polarizing to some with its aggressively triangular grip, but since I hold my pens with the intended tripod grip, I don't have problems with it. That being said, if you're a lefty or don't hold your pens normally, the Lamy Safari might be uncomfortable for you. The good news is Safaris aren't hard to find; go try one out in person for yourself and see how they feel in your hand.
I love both my Safari & my Kakuno. Both are durable and well-made, with dependable nibs that have never skipped or hard-started (when your pen doesn't produce ink immediately as you start writing) on me. The only big differences between the two for me are the clip the Lamy has that the Kakuno lacks, and the more extreme grip on the Lamy.
Finally, my Kaweco Sport is the newest addition to the collection. I love its small size and the sheer variety of colors & materials you can get it in. It's extremely light and very comfortable to write with when posted. But the nib is not nearly as good as the Pilot and Lamy ones I have, and it often hard-starts (though that may be because I got a Medium nib and not a Fine like I did on my Lamy). The screw-on cap (as opposed to the ones that click on the Lamy and the Kakuno) gives me peace of mind when traveling, but screwing the cap off & back on again can get annoying if you start and stop writing often. I would recommend the Sport if you want a small, compact pocket pen to take around. If your fountain pens will mostly be staying in one place, I recommend the Lamy Safari and the Pilot Kakuno over the Kaweco.