The most important thing is to not go for options like Google (8.8.8.8) or Cloudflare (1.1.1.1), which a lot of techies default to.

Using your ISP's DNS is actually relatively okay, because they are quite well regulated by the GDPR and ePrivacy Directive (e.g. they cannot sell your traffic data or use it for advertising without proper freely-given consent) and you're already paying them so they don't need to sell your data to turn a profit. In most cases this configuration is good enough.

The remaining issues could be mass surveillance (some EU member states force ISP's to keep traffic logs for fighting crime). Switching to a third party NS recursor could work, but you would then have to trust them.

Or perhaps you want DNS over TLS or HTTPS, which not all ISPs offer. Without that, DNS is unencrypted so an wiretapper between you and your ISP could monitor what websites you visit. But such an attack isn't very likely to happen.

Lastly, some internet censorship is done by forcing ISP's to block domains at the DNS level. Using a different DNS recursor gets around that, as long as there are no more sophisticated blocks in place.