Usually just good fresh lean meat with enough fat for juiceiness. I have sometimes made them more like a meatball or meatloaf recipe with seasonings, egg, crumbs, etc-- they taste good, but tend to stick to the grill and fall apart. I think plain is best-- add the condiments (onion, pickle, tomato, lettuce,ortega chili, cheese etc.) after it's cooked. (OK-- it's California style)
Fresh ground beef, soaked white bread (white and one slice), pressed garlic, salt, pepper, fresh oregano & basil, soya sauce, drop of oil all mixed together and made into patties. Cooked over the grill - high heat to sear the outside, then medium heat until medium well.
Side dishes and condiments: sweet honey onions (recipe is published), tomatoes, pickles, hot peppers, shredded lettuce, sauteed mushrooms, dijon mustard and anything you like
I know this sounds weird, and most of you won't even try it, but spreading a thin layer of clay-type, unscented kitty litter on the outside of each patty produces an incredibly juicy and tender burger.
The clay quickly hardens and encases the patty which seals in the juices that normally drip into the flames. Simply tap the patty with the side of your spatula after removing from the grill, and the clay casing will cleanly seperate from the burger.
I had to see this to believe it; friends are amazed when I show them this technique and they taste the results.