Buy regular ground beef instead of lean or extra lean. The fat adds the flavor. It's true that simple beef+salt+pepper is often the best way to do it. Just make sure to only salt your patties directly before cooking. If you salt them ahead of time it will draw moisture out and change the texture.
The two things that I feel help my burgers the most though would be 1) digitial scale (so each burger is the exact same size) and 2) a burger press (so you get a nice uniform shape, and the dimple).
I bought one of these about 5 years ago,
https://www.amazon.ca/gp/product/B00...?ie=UTF8&psc=1, and I use it all the time. It was only $13 when I bought it, but it doesn't look like amazon acutally sells them anymore.
Sometimes I'll add a bit soy soy sauce + Worcestershire if I want to pump up the flavor a bit, but I never add egg or onion or breadcrumbs to the patties.