Heat a large skillet over medium-high heat, add ground beef and diced onion, cooking for 5-7 minutes while breaking up the meat until browned and onion is translucent.
Add minced garlic during the last minute of cooking, stirring constantly, then drain any excess grease from the skillet.
Transfer the cooked ground beef, onion, and garlic mixture to your 6-quart slow cooker and spread evenly across the bottom.
Drain and rinse the black beans and pinto beans, then add them to the slow cooker.
Drain the corn and add to the slow cooker, then add the undrained Rotel tomatoes, undrained petite diced tomatoes, and tomato sauce.
Pour the beef broth over all ingredients, sprinkle taco seasoning and ranch seasoning evenly over the top, then stir thoroughly to combine.
Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, avoiding lifting the lid during cooking.
Stir the soup, ladle into bowls, and serve with desired toppings such as shredded cheese, sour cream, avocado, tortilla chips, cilantro, and lime wedges.