There are many choices for DNS providers and the default, using DNS servers from an ISP, is the worst option. Some options are below, the list is far from complete. DNS can be a great way to get ad blocking, tracker blocking and/or malware blocking without having to install software.

Old insecure DNS is specified with IP addresses (normally two of them). New Secure DNS is specified with a server name. Typically a company offers one server for DoH and another for DoT. That said, the two secure DNS flavors use different TCP ports, so they could both be available on a single server.

To test which DNS system/servers your computer is using, see the many available tester pages on the Test Your DNS page.

• The main Quad9 service offers malware protection. More here.
  IP addresses: 9.9.9.9 and 149.112.112.112 | DoH: https://dns.quad9.net/dns-query | DoT: tls://dns.quad9.net
  
  
• Cloudflare offers three different DNS services. The original service does no filtering. In April 2020, Cloudflare introduced two filtering DNS services. See an overview.
  No filtering:     1.1.1.1 and 1.0.0.1 | DoH: https://cloudflare-dns.com/dns-query
  Block malware: 1.1.1.2 and 1.0.0.2 | DoH: https://security.cloudflare-dns.com/dns-query
  Block malware and porn: 1.1.1.3 and 1.0.0.3 | DoH: https://family.cloudflare-dns.com/dns-query
  
  
• My personal preference is NextDNS which blocks ads and trackers. It is a free service, up to a point. You do not need an account to use NextDNS but there are advantages to creating one such as using Secure DNS and configuring block/allow lists. NextDNS allows you to create customized DNS profiles for a group of your devices, for a single device or even just for a single browser on one device. These customized profiles can have their own block/allow lists. NextDNS can also do logging, of both allowed and blocked DNS requests. Setup instructions for all supported operating systems are available on their website after you click on the blue Try it now button on the home page. This generates a free temporary account good for 7 days. The setup instructions will include IP v4 addresses for old insecure DNS. Unlike other DNS providers, these IP addresses seem to vary, but expect them to start with 45.90.
  
  In the below, xxxxxx is the NextDNS profile ID. A NextDNS account can have one or more profile IDs. Generic refers to all devices/browsers that share a profile ID. Customized refers to naming a specific device/browser within a given profile. Customization is very useful when logging DNS requests.
  
  DoT Generic:  xxxxxx.dns.nextdns.io  |  Customized: MichaelFirefox-xxxxxx.dns.nextdns.io
  DoH Generic: https://dns.nextdns.io/xxxxxx | Customized: https://dns.nextdns.io/xxxxxx/MichaelsLaptop
  Chrome browser -> Use Secure DNS with Custom: same as DoH above
  Firefox browser -> Enable DNS over HTTPS with Custom: same as DoH above
  Android Private DNS Generic: xxxxxx.dns.nextdns.io | Customized: MichaelsFone-xxxxxx.dns.nextdns.io
  
  
• VPN company Mullvad offers two free DNS services to the public, as well as to their customers. One service is unfiltered, the other blocks ads. Each service is offered three ways: by IP address, DoH or DoT. Note that their Secure DNS server names are the same for both DoH and DoT (despite "doh" being in the name). This is possible because DoT uses port 853, while DoH uses port 443. This article of theirs has setup instructions for Firefox and Android DNS over HTTPS and DNS over TLS (last updated Feb 15, 2023). The article also explains how to test that their DNS system is actually being used. Mullvad customers can add tracker blocking and malware blocking to the normal ad blocking on Android with the custom DNS option of their app and specifying IP address 100.64.0.7. In February 2023, they added a way to use their encrypted DNS service on macOS, iPadOS and iOS as per this article of theirs: Profiles to configure our encrypted DNS on Apple devices. On these systems, you have to configure a "profile". Mullvad offers the text of the profile file, but no setup instructions. Everything to do with DNS is a pain in the neck on Apple devices.
  
  No filtering: 194.242.2.2 and 193.19.108.2 | DoH and DoT: https://doh.mullvad.net/dns-query
  Block ads:   194.242.2.3 and 193.19.108.3 | DoH and DoT: https://adblock.doh.mullvad.net/dns-query
  Android Private DNS: specify without "HTTPS ://" in front and without "/dns-query" at the end.
  
  
• OpenDNS offers some malware protection by not resolving/translating known bad website names. Their standard service IP addresses are: 208.67.222.222 and 208.67.220.220
  
  
• AdGuard offers both free and commercial services and the line between them is confusing to me. They offer three DNS services, the main one blocks ads, tracking and phishing. Their Family Protection service does this too and adds the blocking of adult websites and a Safe search. They also have a non-filtering DNS service. They also offer installable ad-blocking software for Windows, Mac, Android and iOS. Their AdGuard DNS is in beta as of March 2022. For more see Connecting to a public AdGuard DNS server.
  
  Blocks ads, tracking, phishing:
    IPv4: 94.140.14.14 and 94.140.15.15
    DoH: https://dns.adguard.com/dns-query
    DoT: tls://dns.adguard.com
  
  Family Protection
    IPv4: 94.140.14.15 and 94.140.15.16
    DoH: https://dns-family.adguard.com/dns-query
    DoT: tls://dns-family.adguard.com
  
  
• Control D is a new service (released in 2021) from the developers of Windscribe. There are free and paid services and good luck drawing the line between them. There are about six standard configurations plus you can create a custom configuration. Quoting: "CONTROL D is a fully customizable DNS service, similar to Pi-Hole, AdGuard or NextDNS, but with proxy capabilities. This means it not only blocks things (ads, porn, etc), but can also unblock websites and services." More here. Their standard configurations include: no filtering, filtering malware, filtering malware, ads and tracking, filtering malware, ads, tracking and social, filtering malware, ads, tracking, Adult Content and Drugs. See too their blog Why You Should (and Shouldn't) Use Control D (June 2022). This may well be a fine service with many features (I have not used it), but I don't think they can explain it to non techies.
  
  
• On iOS consider the Privacy DNS app by Disconnect. It is free and blocks trackers and ads. It also does encrypted DNS.
• For a longer list of DNS providers, see Known DNS Providers from AdGuard

AD BLOCKING TESTER

If your DNS provider offers ad/tracking blocking, then you can see how well it is working at this tester page d3ward.github.io/toolz/adblock.html by Eduard Ursu. Note that it is possible your web browser is also doing some ad or tracker blocking of its own. For example, ad blocking is built into the Brave browser and its available from many browser plug-ins.