Ricotta is definitely the best choice for this dish, but if you can't get your hands on it the closest substitue in terms of flavor is cream cheese. The difference is that ricotta is made only from milk, cream cheese also uses cream. It's also more dense, so will require a bit more arm strength to mix into your sauce. In terms of texture the two best substitues are soft goat cheese or sour cream. If you're not a ricotta fan it's not something you taste strongly in the dish, but you may want to opt out of the dollop on top. You could also replace it with mozzarella ovoline balls or mozzarella pearls for a similar serving presentation.