hI've owned several different Waterman Ideals from the early 20th century, a couple of Parker Vacs, 2 Wahl Eversharp vintage pens, a few Esterbrook, and a few Sheaffer Balance, so you could say I've had my fair bit of experience with vintage pens. Honestly, I hardly ever used the flex feature of any nib. It just doesn't suit my regular cursive handwriting. So that feature is meaningless to me. Here are a few other observations as to why I will never go back to vintage.
1. I think most vintage pens are ok, but the hype based on nostalgia plays them out to be something greater than they are. There is a reason why hard rubber is not utilized on top pens anymore. Hard rubber oxidizes, sometimes at an alarming rate, and while you can treat it the pen will always be prone to oxidizing again. Second, rubber sacks are a pain to deal with when they fail, and you never know how much ink the pen really has (running out of ink sometimes at inconvenient moments.) Also, some of the more fragile plastics used in the 1930's and 1940's are prone to shrinkage, discoloration, etc. A modern pen from a high end manufacturer like Sailor, Pelikan, Pilot, Aurora, etc., is simply better than vintage. The plastics are more stable, most have ink windows or ways to easily determine ink levels, the nibs are comparable to vintage non-flex nibs, and modern materials are less likely to fail in the way vintage materials will.
2. Modern filling systems are more convenient. While I dislike CC fillers, they are super fast to refill and most importantly the process is not messy. Modern piston filler systems (my preference) and even modern vacs are sturdy and reliable, and you don't have to deal with the drawbacks of vintage piston fillers or vac fillers with cork seals that might have gone bad and are ready to spill ink all over your pocket or carpet, or leave you unable to refill the pen when you need it. And let's not even get started on "safety pens." Those old safety pens are just waiting for one unguarded moment of carelessness on your part to give you, your possessions, or your dog sleeping by your feet an ink bath.
Personally my ideal pen these days is a Pelikan M200/400, M600, or M800 (I love them all). Aurora Optimas are also great pens, and the Pilot Custom 823 also ranks high in my book. I've also tried the Lamy 2000, and while not my personal favorite in the style department, it is also a fantastic pen which I would not mind using on any occasion.
I think you confuse a few things here. It's perfectly fine to prefer the writing experience of a modern pen. And that's what the question was about, the writing experience. Others may prefer how a particular vintage pen writes. The same person might not like the feel of a different vintage pen at all. My opinion is that there is simply no ranking because it's just a matter of personal preference.
But you seem to focus mostly on things like material stability or leaking, which first of all wasn't the question of the OP and second is not necessarily a question of vintage vs. modern. People in the 50s did not run about ink stained every day because they were condemned to use a period pen. And the number of posts here asking about various problems with their brand new pens, including leaking, does not shine a good light on current products.
Just to address some of the points you raise:
It depends on the manufacturer. Top tier HR pens 100 years old stood the test of time surprisingly well. Third tier pens may crumble by now. Are current materials run strong for 100 years? Maybe, maybe not. Furthermore, I think that some current luxury brands do have HR pens in their portfolio.
Sac fillers do have disadvantages for sure but they are the quickest and most convenient to fill (maybe except for a cartridge filler). The sac is a consumable and must be replaced every few years. But that's no different to the tires of your car. If they are maintained properly, there is hardly a risk of leaking. At least I had non so far. And guess what, some super expensive current luxury pens fancy a sac filling system.
Yes, some of the old thermoplastics were cumbersome. But in my experience most are not and are still in use. In my extended collection of pens from the 1920 to current I see little evidence that top tier vintage materials show more deterioration than current ones. I have Pelikans from the 1930s and 1940s that are in perfect shape, so are Montblancs from the 1940s and 1950s, OMAS from the 1930s and 1940s, Osmias and Kawecos from the 1930s to name just a few from here. My Parkers from the 1940s did not age so well but my Wahl Eversharps and Shaeffers did. By contrast, my first modern Montblanc bought in the late 1990s was a total letdown and broke down leaking repeatedly, despite Montblanc service.
Except my Pelikan and Montblanc current pens, none has an ink window. But Pelikan and MB always had ink windows or transparent barrels. Most other vintage piston fillers also fancied ink windows like Osmia, Kaweco, Geha, and countless second and third tier German producers.
Vintage piston fillers:
If you buy a 1950s vintage car, you better make sure that the tires are new. You also may want to check a few other technical details. Same with cork piston seals. A pen that fell dry for decades just needs a replacement and then it's good to go for another few decades of continuous use. But even better, synthetic seals have been introduced in the 1940s, e.g. by Pelikan, and those often are still good to go. My Pelikan 100 from 1942 war time production, probably the first pen to have such a seal, does not leak whatsoever and is in original condition with nothing replaced as far as I can tell.
So, long story short, I think it's all about personal preference and the feel of the nib.
Edit (reason: Alzheimer):
I forgot to mention the nibs. If you prefer a firm, smooth writer, there is little need to go vintage because there are many excellent contemporary offerings. You still could go vintage in order to save money, though. But if you seek a more responsive, more flexible nib for whatever reason, may it be the freedom to change line variation at will or just the pleasurable feeling of such a nib, then the current productions have little to offer and you might be better off with a vintage pen.
Edited by OMASsimo, 19 March 2019 - 22:08.