In addition to 100% beef (no additives or other meats added), the most important factor is fat percentage. Ground beef can be 75%lean/25%fat, 80/20, 85/15, 90/10. The fat level contributes a lot to the flavor of the finished product. Many burger experts recommend 80/20 for that reason. OTOH, more fat means more shrinkage and more flare-ups.
Ground beef is seldom sold pre-seasoned. You should plan on doing your own seasoning. Ground chuck is a good choice. Chuck is a flavorful cut of meat and has a good fat percentage.