I couldn't choose a favorite fiber.
Synthetic tufts have reached a very high quality, they cost relatively little, they don't need maintenance, they dry quickly, they are ready to use and they easily mount any type of soap.
Boar tufts are very cheap, they have a massage and a pronounced backbone and they need the right amount of re-wetting to perform at their best.
Badger tufts are the tradition, they are on average more expensive than the others, they need more care and constant maintenance, they dry more slowly and the animalist impact they produce should not be underestimated.
In any case, at the moment and according to my preferences, they are still the best brushes that can be used.