I need to have a robust database with my app, what can i use

Can I use My SQL, Csv etc as database. what is bets database to use with app inventor

You have plenty of options, it will come down to what works best for you in your given situation and for your app.

Look in Frequently Asked Questions category (and search forum):

CloudDB (and variants - redis servers)
TinyWebDB (and variants)
Google Sheets
Firebase (realtime/firestore)