There are several kinds of avocados which come from various countries, but to be really good they must be perfectly ripe, i.e. they must "give" a little, neither hard nor too soft.
To check this, you should never press with your thumb on the fruit, because you will bruise the flesh and make a brown mark which will show after peeling. The simplest way is to take the fruit in your hand, and exert a slight pressure with all your fingers. You should feel a certain flexibility.
If it's completely soft or very hard, look for another one. If it's firm, buy it, but in this case leave out of the fridge for a day or two so that it can ripen fully.