As with any standardized test score, the higher the score, the better. But that’s really not a very helpful answer, is it?
Each school will have its own cutoff score that it wants to see from you, and each school will weigh other elements of your application in addition to your ISEE score. Your race and gender will also affect how schools view your application, as will your family’s economic status.
So there is no single “good ISEE score” that will guarantee you can get admitted to every school you want. One of the easiest things you can do, strategically, is call up the admissions offices of the schools you’re applying to, describe your situation, and ask what kind of a score they would consider competitive for someone in your situation. Another approach is to find out the scores of people who’ve been admitted to your target schools in the past either by calling up the school or through doing online research (remember to make sure you’re comparing yourself to people with similar genders, races, and income levels for the best indication).