Mmmm, the perfect burger, let's see...
It depends on the day. But generally I buy 80/20 bulk and mix it well with a seasoned panade (bread/milk paste) before forming patties and freezing them. I prefer medium well burgers with hearty crusts, so the panade lets you keep it on the heat longer without getting tight and dry. I'll change up seasoning every time, but I never use salt when making patties. I only salt and pepper the burgers a few minutes before they hit the grates. If you put it in the patty itself, it will change the texture for the worse.
Occasionally I'll "grind" up my own chuck in the food processor, but that makes more of a steak burger. Some day I'll get a Kitchenaid mixer with a proper grinding attachment. Fresh ground is really about as good as it gets.
Cheese is nearly always American. There really is no better melter with just enough salt. I prefer white "deli style" slices. I'll fold in the corners so they don't melt away into the grill. I care about my cheese. I want to eat it all. I'll often augment the American with another... bleu cheese is amazing.
Bun is either a Marty's giant sesame roll or a Martin's potato roll.
Most ubiquitous toppings for me? Definitely mayo (on the bottom to absorb grease) and lately a lot of this stuff:
The marriage of that and mayo on a burger is absolutely divine. Lettuce, onion, and tomato are all great but not required once you get that mayo/chili garlic thing going on. All sorts of mustards also welcome.
Oh man, is it too late to make one? Never...