Avocados ripen best off the tree, so if you can wait to use them, buy the rock hard ones. The best are the purplish black, bumpy-skinned Haas avocados. They have a rich, buttery flesh. The smoother, green-skinned California and Florida avocados are too watery, too sweet, and tasteless. It is preferable to use Haas avocados in Southwestern and Mexican cooking.
Prevent avocado flesh from discoloring by brushing it with lemon juice as it is peeled. It is a myth that by burying the avocado pit in the guacamole that it will prevent it from browning.