Salted or unsalted butter
It really depends on how much salt you like in your food.  Most recipes are written for unsalted butter because it makes it easier to control the amount of salt.  If your butter is salted, you can use it and it will probably still taste good.  Although, you may need to reduce salt a bit elsewhere in the recipe if it uses a lot of butter.  When in doubt, go with unsalted, then add salt as needed.