While I haven't tested it, I do understand that most likely the best modern flex nib is the Montblanc Calligraphy nib. But pens go for up to 1000+ USD, so you probably do not want to go that route now.
As for the other pens... I understand a Pilot pen with a Falcon or FA nib may run easily for 100-200 USD. At that price range you can find vintage flex nibs that may be better (sorry, no experience with the Pilot nibs either so I cannot compare). You can get a Mabie Todd for that or even a lot less. If you look in German eBay, you can find many piston fillers with flex nibs in price ranges around those of a Noodler's Konrad with music nib (search for "füllfederhalter flex" and look at the vintage ones). My most beloved flex pen is a Salz Bros Peter Pan that cost me some 20-30 USD.
As for the rest... I'd venture to profer the following advice (but do not take me too seriously, I'm by no means a real expert and most of what I say is hearsay): from my point of view you have two avenues to follow.
The 'short term cheapest' is to get a Jinhao (or similar) and swap the nib for a Zebra G nib. This would give you an idea of what a great flex nib for an FP might feel like, if you can find one as flexible and thin writing. The downside is the nib will last a few weeks and you'll need to swap it again, so in the long run may turn more expensive (but also lots of fun).
The longer term, cheap, is to go for a Noodler's pen (if you can get a Konrad with music nib that may be nice, but a Creeper or Ahab will do as well with a flex nib after applying the Ease My Flex mod, look up the thread here on FPN), or a cheap -yet compatible- pen with an FPR flex or ultraflex nib (If you get the FPR flex nib, you'd apply the Ease My Flex mod to it, the UltraFlex is just a flex with the mod already applied, in my experience, a #6 nib has been more flexible and manageable). The downside is that it won't be as flexible as a Zebra G, but it will not wear out (not so fast at least).
Note that modern flex nibs (except perhaps for the MB Calligraphy nib) are actually semi-flex nibs and require a significant amount of pressure to flex, and so may result tiresome for long writing sessions (which is the reason for the Ease My Flex mod or buying an FPR UltraFlex, and for swapping in a temporary Zebra G as well).
To sum it up: what I did was test first a Jinhao with a Zebra G, then I tried the FPR flex nib and Noodler's nib creaper, then I applied the Ease My Flex mods and finally I decided to explore the vintage route, which is -I would say- the most satisfying. But, OTOH, how can one know if going vintage is any good without having tried first modern semi-flex to be able to compare?
Edited by txomsy, 19 December 2019 - 20:22.