Typically you want to use an 80/20 blend for a juicy burger, especially if you are grilling, because some of the fat will drip off. If you are using a pan to cook your burgers, you can use a leaner ground beef, because the patties will be cooking in the grease. That being said, I've used 90% lean ground beef for burgers, and they are still mighty juicy with the addition of worcestershire sauce!