Ice is one of the most important ingredients both in cocktails and elsewhere. Even mineral water improves in taste with the addition of ice, and the process of drinking becomes more aesthetically pleasing.
For some reason, ice is too frequently glossed over. It's important to remember: if you're planning on putting effort into making a drink and you strive to choose only the best ingredients, then you need to start with good ice.
Ice is the only ingredient of the cocktail which has one, even two specific tasks: such as cooling and diluting a drink.