IMO, the best free option is https://freedns.afraid.org/. The biggest downside of that one is that you have to login a couple times a year (IIRC?) to keep it active. I actually still use this even though I have a paid domain, I just CNAME my real domains to the afraid dynamic name. That was easier than changing the config every time I become unhappy with my domain registrar and have to reconfigure everything after swapping.