Presumably, you like all monkeys and all dolphins (or at least the majority), so you should use the plural form. If you are being specific, stay singular; if talking about a group, be plural:
My favo(u)rite actors are Jackie Chan and Jet Li.
My favo(u)rite colo(u)rs are red and blue.
My favo(u)rite countries are China and Japan.
My favo(u)rite animals are cats and dogs.
My favo(u)rite cars are expensive (ones) and fast (ones).