The choice of database for a Xamarin app depends on the specific needs of your project. SQLite is a popular choice for local data storage due to its light footprint and the availability of libraries that make it easy to interact with in C#. It supports most SQL syntax and can handle complex queries. For applications with real-time data requirements, Firebase can be a good choice due to its real-time database and plethora of other useful services. If you need offline sync capability, Azure Mobile App Services provide this functionality along with a scalable backend. If you have complex data needs, you might also consider NoSQL databases like MongoDB or Couchbase.