Beef or chicken
Anything braised is always better the next day. Short ribs, coq au vin, beef stew, brisket...
Beef or lamb stew, roast chicken, chicken cacciatore, any pasta dish that mixes in chicken, chicken sausage, or pork sausage, lasagna, shepherd's pie, meatballs, meatloaf. You might want to try going to the Recipes link at the top of this page and doing a search on the word "casserole." Then use the categories on the left to narrow down your search.
Beef stew (Paula Wolfert has a great Moroccan beef or lamb tajine with prunes, cinnamon, and honey-glazed apples in her cookbook, Couscous and other Good Foods from Morocco), braised chicken (coq au vin or chicken in a tomato-based sauce--a Georgian friend of mine makes a delicious one with lots of garlic and cilantro that is falling-off-the-bone tender), braised veal with mushrooms and white wine (I make this recipe quite a lot, it's good even without the gremolata, and I often use fresh instead of dried mushrooms: http://www.epicurious.com...). Soups are always better the next day; tomato sauce for pasta can be made the day before, and the pasta boiled the day you plan to serve (or, as the case may be, eat!). Anything liquidy and simmered/braised benefits from allowing the flavors to meld overnight.
Oh! I forgot to add chili. Good the first day, even better the next!