Lean ground sirloin, grated onion, salt.
For each pound of ground beef, add 1 teaspoon of ground cinnamon. Sounds weird, but it gives the burgers an nice flavor, and you can't tell what causes it. (This works for meatloaf too)
Make the burgers in the normal manner. I like to add finely minced onion to the ground beef. I will sprinkle with salt & pepper as the cook. Simple & delicious.