What DB and Why?

Whichever one you're most comfortable with.

Seriously, though, while the fanboys may say MySQL is over, it's not. MySQL is still the king of the web. Percona is a drop-in replacement for it that can run faster and use less resources.

Postress has some very unique features and is a very good option. Also fairly decently supported in hosts.

NoSQL works fantastically for some things. The performance hype that I've seen though, didn't hold up in my experiences with it, if you needed to have 100% reliable data integrity it came in close to MySQL speed. It does require a slightly different way of thinking about your data, though, and I found it to be easier in some ways, and more difficult in others.

Really boils down to your app's needs, and hosting requirements, and what you and your team already knows, or how much time they have to learn something new.

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