To build a blog website, what is the best database to use?

MySQL

It depends on what programming language you use.
Usually SQL Server used -but not mandatory- when ASP is used, MySQL is used when PHP is used.
MySQL and SQL Server are the most common in web-based application.
Saad,

MySQL is the most common database used for blog sites. Reason being that it is optimized for read performance and on a blog site, reads from visitors occurs more frequently than writing which occurs only when a new blog is posted. PostgreSQL is also getting traction as it has more features.

Oracle

MySql