Most basic. Ground beef with store bought seasoning packets and water. Step that up by adding onion, peppers, chilies, tomatoes. My dad's variation was to add shredded potatoes. Stretched things on a tight budget but also tasted good. My favorite is to make Cuban Ropa Vieja and use it as taco meat. Great way to use a real cheep roast. Close second is slow cooked pork with basic Mexican spices. Then there are all the seafood meats but that could be a second thread. Tempura fried shark meat makes awesome fish tacos. What ya got?