Pick anything but what you'd order in a steakhouse. You want meat that's tougher, with bones, tendons, and marbling. (No thick fat around the edges, though.) The more gristle, the more it melts into delicious gelatin, so go for chuck, shoulder, rib, brisket, or belly cuts of beef, pork, or lamb; bone-in legs and thighs for birds.
This is everything you add for flavor (not just scent). Vegetables bring sweetness and melt into the sauce for texture, too. Tomato paste is classic (seasoned with pimentón, it takes on a barbecue-sauce-like flavor), or try mushrooms and peppers with herbs and spices. You can even flavor meat with more meat: Feel free to drop in some salami or bacon.
Any stock will work, but if you have plenty of good ingredients, you can make a credible stew even with water. Beer and wine are also great sources of flavor. Avoid salty canned broths or very hoppy, bitter beers – cooking concentrates flavor, so everything that goes in is stronger coming out.
Most sauces like something at the end – fresh herbs, a pinch of salt if the flavor seems thin, a few pats of butter if you like it richer. Add lemon or vinegar for brightness or a little sugar or honey to round it out. Then add garnishes for texture – fried onions for crispness, diced or shaved raw veggies for snap, croutons for crunch.