Probably merch is most effective these days? You can download a mp3, but you are never able to download a T-shirt.
But at same time, merch have higher entry gate. You are unlikely to buy a merch created for someone you don't really know. (Unless it is just good enough that you are willing to just use it like a general T-shirt...etc)
Performing has always (and should always, if you ask me) be the main avenue to make money for musicians. Music can be copied, can be covered, can be mixed etc. Chords can be guessed, copied and stolen.
But the musician herself is irreplaceable. If you want to make money by making music, be sure to develop a relationship with your audience rather than police it in order to ensure you're getting paid.
But at same time, merch have higher entry gate. You are unlikely to buy a merch created for someone you don't really know. (Unless it is just good enough that you are willing to just use it like a general T-shirt...etc)