For a lot more flavor with just a little more fat, select 85% lean ground chuck. I've found that 80% lean makes tasty grilled burgers but it can also cause grill flare-ups from fat drippings, as well as noticeable shrinkage during cooking. On the other hand, I find hamburgers made with 90% lean to be pretty dull. A certain amount of fat brings additional flavor to whatever you're cooking, and the meat for burgers is no exception.