Well I'm not a chef but I make a damn good burger and never ever dry, maybe because I put wine and BBQ sauce to prep it? I just don't like the taste of fat and grease, it gives it like a dirty taste to me.
I'm just telling you what was told to me by professional chefs. I work in film and television and we did a project with 6 top chefs, couple had been on iron chef, few others have their own shows, most had their own restaurants, james beard award recipient in the mix, and the question came up and was answered, start with 80/20, agreed to a man, you need the right fat to protein ratio to yield moist flavourful results. Now if that is not your preference that is fine but by and large, 80/20 is the direction a chef would go. Another guy might think that a super lean cut of steak is the best tasting to his palate but somebody who cooks them for a living will tell you, you want high quality marbling. Theres a reason waygus that are the most marbled and score above 9 are crazy expensive but doesn't mean some guy isn't gonna tell you his lean costco cut done his way is the most tender piece of meat you'll ever eat and it may be to him. There isn't an absolute here, just what works for ya and your individual preference, and if you can make 90/10 work for ya, great, but outside a joint catering to bb'ers, most chefs aren't sending out a burger to customers that is close to that fat percentage. Fat is usually flavour to the majority.
My favorite is my own burger. I have a huge pattie that I cook until it is med-rare, then I top it with cheese, pickles and ketchup. Thats it. I know it doesn't sound like much but I love it way better than anything I can buy in a restaurant.
In SF the best burger in town is at Holy Grill. They have various kinds of beef. You can order extra chunks and it tastes delicious without a lot of extra ketchup or mayo. I generally love adding tons of mayo since it's fatty and doesnt spike blood sugar