Rock and Andy; you guys impress me every time you speak. I want to add this to what you say. Some beef cuts have a more pronounced "beef" flavor than do others. Chuck, sirloin, and round have good flavor. Shank has even more flavor. That flavor comes from muscles that are well exercised. Ever eat beef heart? If you have, then you know what I'm talking about.
On the charcoal grill, fat is important for its ability to melt, and drip onto the hot coals and make smoke that flavors the meat. It also has a flavor of its own. But that flavor is very mild. In a pan fried burger, it contributes more to texture and how juicy the burger is.
For a pan fried burger, I prefer an 80/20 mixture for the richer beef flavor it has. The best burger, IMHO, comes when you choose a suitable cut, and smell it. You can tell by the odor of the raw meat how much beef flavor it will have. Then, you have the butcher grind it with the proper amount of fat. If you can't do that, take a whiff of the pre-ground beef that you are purchasing. Check the ground beef quality of different stores. See if you can find a store that will grind it for you.
I once had an amazing burger in this little corner restaurant, located in Olympia Wa. It was cooked medium rare, and tasted more like a really great flavored steak than it did a burger, just not as expensive.
I guess what I'm trying to say is that "great burger" means something different to everyone. For me, that traditional pan-fried, or grill-mark burger flavor is a combination of good tasting meat, with the flavor imparted by the browned meat and salt, and the textural and flavor qualities of the contained fat. Charcoal grilled traditional for me is that great flavor, in the proper amount, provided by quality meat, juicy texture, and smoke from burning fat.
My ideal burger, which I have yet to be able to duplicate, is that fire-grilled, steak-tasting burger that I had in that restaurant.
As with all food, it's important to understand the role of different ingredients in the food we make, as well as how the techniques we use react with those ingredients. I think of ground beef as an amalgamation of lean beef, fat, and seasonings. I then use different techniques to cook it for the purpose I'm using the ground beef for. Pan fried, or pan-grilled, broiled, flame grilled, barbecued, browned for use in a sauce, as an ingredient in meat loaf, or soup, or tacos, or whatever, understanding ground beef by its ingredients, and how to choose the correct technique for the recipe you are making, is key to success.
Andy & Rock, you guys are among the most knowledgeable cooks I know. And I'm still learning from both of you, and hope I am contributing to that pool of knowledge that we all can draw from.
I'm an analytical cook. It helps me understand, and engineer foods to be what I want them to be. And to the OP, I hope you are able to glean something useful from these posts. Good luck in your quest for the perfect burger.
Seeeeeeya; Chief Longwind of the North