Meat mixture? Just meat that is at least 20% fat. Anything 20-30% fat is fine for me. Hell, even fattier is okay. If you want to get more serious, then any mix of ground chuck, short rib, or brisket. I love pure ground short rib, but it’s quite rich and fatty.
Don’t add a damned thing to the meat. You’re not making meatloaf. None of this eggs and breadcrumbs shit. I mean, if you like a meatloaf sandwich, go for it. Just don’t call it a hamburger.
Form a loose patty. Just enough to bring the meat together. More on this in a second. Salt and pepper generously. You can also add onion powder or garlic powder or whatever you like. These days, I’ve been doing salt, pepper, onion, and cayenne.
For pan fried burgers, I like them thin, like anywhere from 1/8 to a quarter pound. Make a loose ball of meat (like I said above, don’t overhandle), place it between two sheets of wax paper, and smack down with a heavy object (I use a small cast iron pan.) Season as above. Get a cast iron hot pretty much blazing hot (medium-high heat to high) and fry about 1 1/2 - 2 minutes per side. You want it to crisp up and fry. With this thinness, you’re going to be ipretty close to well done territory, but it’ll be fine, as you’re developing a nice brown crust and all sorts of Maillard-y goodness, and the looseness of the patty and thinness will keep it from feeling hockey-puck-like in the mouth.
Now, if you like it thick and don’t want to do it on a grill (for grilled burgers, I usually do 1/3 lb - 1/2 lb patties), this is what I do if I want them rare to medium rare. Set oven to 275. Make the loose patties, no dimple required, and season well. Cook for about 20-25 minutes. You can use a thermometer and pull them when they read somewhere around 115. Get a cast iron pan nice & hot, and finish them 2 minutes a side on the pan. With a grill, it’s a similar technique; I start them on the cool side, and finish them with the heat maxed out on the hot side.
There really is no trick to it other than: use meat with a decent amount of fat, handle the meat gently, season generously, and cook them intelligently (using the techniques above, or similar, for what style you’re going for.)
The thing about handling them gently is that you want the looseness of the meat and the air pockets to be maintained when you cook the burger. This is part of the loveliness of a great burger–it holds together but its almost falls apart in your mouth as you bite it. You are not making a sausage where you knead the crap out of your meat to achieve a springy texture. Be gentle with your meat. And please, please, please for the love of god, no binders or fillers.