Depends. If it's a roast of course but if we're using a pan it's whatever the fat you chose to cook with. Always helps to toss some herbs of your choice and cloves of garlic or whatever suits you. Or you can sear it remove it, put the fat in the pan aside. Deglaze with wine or juice or vinegar or whatever liquid you choose and the slowly reincorporate the fat while whisking and have yourself a fantastic pan sauce. Stay baste.