Which ORM should I use for Node.js and MySQL?

I would choose Sequelize because of it's excellent documentation. It's just a honest opinion (I never really used MySQL with Node that much).

May I suggest Node ORM?

https://github.com/dresende/node-orm2

There's documentation on the Readme, supports MySQL, PostgreSQL and SQLite.

MongoDB is available since version 2.1.x (released in July 2013)

UPDATE: This package is no longer maintained, per the project's README. It instead recommends bookshelf and sequelize