There are two main custom firmwares: Haxchi/Coldboot Haxchi (usually abbreviated as "CBHC"), as well as Mocha.
Haxchi/CBHC are both injected into a paid DS Virtual Console game. Haxchi will need to be manually started off the Wii U Menu to boot custom firmware, while CBHC will boot custom firmware automatically while the system is booting up. Mocha is accessed through the browser exploit or the Indexiine method, which makes it the only free custom firmware.
The custom firmware you decide to use is ultimately **your** preference. There is no right or wrong custom firmware to go with, so it is recommended that you feel free to pick the one that sounds the best for your needs!
<br>- Don't install Haxchi over CBHC! (You will not brick, but it will cause a boot-loop! Hold A when booting to access the Homebrew Launcher and uninstall CBHC.)
> If you are worried about breaking one of these rules while using CBHC, consider setting up Parental Controls for your user! Blocking `Data Management` and setting `Game Rating` to the highest possible rating can help prevent common mistakes users may make when using CBHC. Check [here](https://en-americas-support.nintendo.com/app/answers/detail/a_id/1081/~/how-to-change-parental-controls) for more information on how to change your Parental Controls settings.