The best onion for guacamole is a matter of personal preference, as different types of onions can add different flavors and textures to the guacamole. However, the most commonly used onion in guacamole is a white or yellow onion, as they have a mild flavor that won’t overpower the other ingredients in the guacamole. Red onions can also be used for a slightly stronger flavor and a pop of color, but they may be a bit more pungent than white or yellow onions. It’s best to chop the onions finely so that they distribute evenly throughout the guacamole and don’t overpower the other flavors. Ultimately, the choice of onion will depend on your taste preferences and the flavors you want to highlight in your guacamole recipe.