Cracked ice will give you a middle-of-the-road balance of cooling and dilution. It works best with stirred or shaken drinks, like martinis that need rapid cooling with a little dilution.
Crushed ice cools the drink the fastest, but it also melts the fastest. Best for drinks that could use a little bit of watering down, like a mojito.
Cubed ice melts the slowest and causes the least dilution. It's best with liquor on the rocks, like a whiskey that you don't want watered down.