Beans are the best, as they have the highest percentage of fermentable fiber in the form of raffinose. Pectin in apples, pears and citrus, the beta-glucan in oats and barley, and inulin in alliums, chicory and jerusalem artichoke are all good as well.

To get the most benefit, pair your fiber source with a probiotic food. Good examples are unpasteurized sauerkraut, kimchi, yogurt, kefir, etc.

From https://www.juvmed.com/healthy-diet

Beans, lentils and chickpeas

Wholegrain and wholemeal, bran based cereal, oatmeal

Brown or wholegrain rice

Broccoli, artichoke, avocado, brussels sprouts

Raspberries, pear, apples

Chia seeds