My two cents. Two keys to a perfect "pure" burger. 1) Make you own grind 2) DO NOT season the grind, season the patty just ahead of the cook. Sure sometimes I'm in the mood for something funky and I will add things like seasoning, veggies, cheeses, sauces, whatever to a burger mix.. but this is more a "meat loaf" style burger.
If you want pure, unadulterated, juicy hamburger goodness you can't have that stuff in the mix. You can see for yourself very easily.. add some salt to a grind and cook one burger, just season the patty for another. The difference will be visible.
Of late my grind is equal parts sirloin, brisket, and short-rib. Medium grind, loosely formed patties (another key to juiciness)... season outside just ahead of cooking with S&P. I cook on the grill or cast iron. On the grill, you want it hot to start, then I bump down slightly. I flip once, maybe twice (depending on temp and char). Maybe some cheese and cook to med-rare on a lightly buttered (if grilled), toasted potato roll.