MySQL
MySQL was my first database experience as a developer. Open source and easy to learn, I have used MySQL, or the founder's fork MariaDB, across my career for relational database management systems.
Popularised during the LAMP stack era, MySQL remains a top choice for web applications, despite being seen as less cool. It is supported virtually everywhere and has a huge community surrounding it. It is the default choice for many open source CMS projects like WordPress, Joomla, Magento, among others.
Many successful web companies started out on MySQL, like Facebook, Twitter, and YouTube, before developing in-house solutions at huge scales.