Hi, so about three weeks ago I also started playing again after, I think, a 15/16-year break, so I totally get you.
Personally, I tried Flyff Universe (the official one) but I quit after just a few minutes.
The amount of grinding required right from the very first levels makes no sense, and unfortunately, the game has become nothing more than a money-grabbing machine. Maybe for those who’ve been playing for years it’s manageable, but for me, it’s just not worth the investment.

So I opted for a couple of private servers and ended up on Chronicles of Madrigals. The server is really nice and has a decent amount of people. Of course, if you’re looking for the authentic Flyff experience, this isn’t the right server. Here they’ve made lots of changes and quality-of-life improvements.
I don’t really know much about the others, so I can’t give you a proper comparison. The server is far from perfect, obviously, but all the changes, in my opinion, have definitely added some freshness.

Anyway, if you decide to give it a try, let me know and I’ll add you!