All kind of achieve the same thing, but in different ways.
Pi-Hole is the completely free way of doing ad and tracker blocking at the DNS level. Free as in free beer and free as in free speech.
AdGuard is free as in free beer but not as in free speech.
Both solutions mentioned above have to be self-hosted.
NextDNS is a managed service for which you have to pay a (very small) monthly fee for. The advantage is that - once setup - it pretty much just works (exception being custom updates to filter lists, but that applies to the other two as well). What's cool about that is that it's reachable from outside your local network, so you can use it on your phone or whatever even when you're not at home (they offer apps and profiles for easy setup). You can expose your Pi-Hole/AdGuard DNS to the outside world, but this has some caveats and probably higher latency/worse availability.
Opinions differ when it comes to privacy, but I'd say they (NextDNS) are trustworthy/not selling your data as this doesn't seem to be their business model. Obviously, with Pi-Hole you don't have to trust anyone (except the code authors unless you study the code yourself), so when in doubt Pi-Hole wins in this regard.
Be careful when setting up either of these as the default DNS service in your home network, especially when other users are in your network, as the default configuration of either of these will break some websites, services and apps to stop working and you (the admin) would have to handle the errors your users are getting by adding exceptions and/or different filters. The good news is that there are more conservative filter settings available that will still block most ads and trackers while being way less likely to break anything.