What are the best cuts of meat for grinding burgers?
I use Alton Brown's "Burger of the Gods" recipe when grinding my own burgers:


1 part sirloin
1 part chuck
salt


He uses a food processor and pulses the meat 10x after cubing it. This is much simpler than using the grinding attachment you use; I only use that for some sausages.

The sirloin gives you that rich beef flavor, and the chuck provides the needed fat so you have a juicy and tender finished burger.  I find that when I'm making my own burgers I don't need or want things like onion or other fillers getting in the way of my beef.

You can also find this recipe on YouTube.