Cook the ground beef (or turkey, chicken) in a skillet over medium-high heat. Break up the meat as it cooks. Drain excess fat and transfer to the slow cooker.
In the same skillet, sauté chopped onion and minced garlic until softened, about 3-4 minutes. Add to the slow cooker with the browned meat.
Add diced tomatoes (with juices), tomato sauce, kidney beans, black beans, green chilies, chili powder, cumin, paprika, cayenne pepper (optional), salt, black pepper, and beef broth to the slow cooker. Stir to combine.
Cover and cook on low for 6-8 hours or on high for 3-4 hours. Stir occasionally if possible.
Taste the chili and adjust seasoning if needed. Add more chili powder, salt, or pepper. If too thick, add more broth or water.
Serve chili in bowls, topped with shredded cheese, sour cream, green onions, and cilantro. Enjoy with tortilla chips or cornbread.