I have always had a soft spot for PostgreSQL. It's built for experienced developers to get things done quickly, and it usually out-performs the alternatives.
I recently built a RESTful API to communicate with a mobile application with PostgreSQL on the back-end. It can handle approximately 2,000 requests per second, and making changes to routes, schema and user permissions is fast.
I enjoyed the process enough to post is as a Gig on Fiverr. Check it out if you need a great API!