When it comes to the actual USB loader program, there are two that we recommend: USB Loader GX and WiiFlow. They both have their benefits and drawbacks.
USB Loader GX is a loader that has a design very much like the standard Wii Menu, making it great as a Wii Menu replacement. It also has features such as built in patching for custom online game servers, themes, cheat codes, disc ripping, passthrough for GameCube loaders, and more. This loader is very commonly used.
WiiFlow is the much less commonly used choice for a USB loader. It does, however, provide a couple features that USB Loader GX doesn’t. Loading Wii games from an SD card, it has a much more configurable cover layout (i.e. you can customize your games to look almost any way on screen), and a much more basic and fast UI. It’s not great as a Wii Menu replacement, due to its bland and basic look, but it offers everything USB Loader GX does with a simpler layout.