My best score on the SAT is a 2170. I plan on applying to very competitive schools, including Stanford, Duke, and Northwestern. Should I retake the SAT, or is my current score good enough for admission?
Your score is below average for Stanford and Duke and exactly average for Northwestern, so you stand to benefit from retaking the test. The average SAT scores for the schools you mention are 2210 at Stanford, 2250 at Duke, and 2170 at Northwestern.
If you choose to retake the test, you should put in at least 10-20 more hours of prep time beforehand so that it doesn't end up as a wasted opportunity. Make sure you're scoring consistently in the 2300 range on practice tests before you attempt the real SAT again. It isn't impossible to get into these schools with a 2170, but you'll be safer if you increase your score by 100-200 points.