We use hamburger. I know some people use pork (shredded). I suppose you could use a flank steak or something similar and cut in very small pieces. But IMO that would be more work to cut the steak up.
I use a package of Morningstar Farms crumbles and 2 Morningstar Farms grillers (patties) crumbled up to make 16 oz of "meat." I also use a can of pinto beans, chili powder, cayanne pepper, salt, a can of diced tomatoes and 12 oz of beer.
So far, everyone loves it and no one has complained about the lack of meat.