I've experimented with this for years. The best I've come up with is equal parts chuck, brisket, and oxtail.
But oxtail is now expensive, and it's a huge pain to prepare for grinding (mostly bone). So now I use equal parts chuck, brisket, and shin. Shin is much leaner than oxtail, so I add in the marrow from the shin bones.
I also up the fat a bit with 5% butter, but this is optional. I mix in 0.7% salt and some black pepper.
Even more important than the meat selection is that you grind it fresh, shortly before cooking.
This blend works because chuck tastes like what we expect from a burger; brisket brightens and freshens the flavor (especially if it's pastured beef), and the shin adds super intense beefiness.