They all have different pros and cons.
-I personally mostly use
WiiFlow-mod (which is now the mainstream WiiFlow version) because of the retro emulator support and it just looks cool.
-I also have
USB Loader GX installed because I like the banner player and the fact that it combines multiple partitions and game types in one.
-I've never used
Configurable USB Loader (CFG) but it's supposed to be really good. It scores the best on some
old polls I found.
-I've also heard good things about
PostLoader3.
-There are those that like
NeoGamma because of the simplicity.
-I almost can't justify it any more but I also have
uLoader installed for the external USB DVD reader support and older NAND "emulation".
-There's also always the option of just loading the games in a
NEEK environment.
I think those are the main ones at least nowadays.
Someone should really update the
USB Loader Comparison Table so we can see the pros and cons better. It's really outdated at the moment.
Everyone has their favorite they've used but I don't think there are many that are really well versed in the latest version of all of them.