There's a direct correlation between kitchen temperature and the host's composure (see overheated party thrower syndrome), so it's a big help if you can avoid turning on the oven when your house is packed with guests. Plus, serving room-temperature dishes means you can put the food out whenever you want (not just when it's hot) and not worry about whether everyone is making their way to the buffet table in a timely manner. Your best bet is roast chicken or pork tenderloin, both of which you can cook in the morning and let sit under tented aluminum foil for a few hours. Slice the meat just before serving so it stays juicy. A good vegetarian option is quiche or frittata.
Get the recipes: The Tastiest Roast Chicken Recipe Known to Man, Kitchen Workhorse Pork Tenderloin with Asian Glaze, and Zucchini Frittata