I've looked at MySQL, Postgres, Oracle, Couch, and many other database applications over the past few months.

I like Couch the best.

However, I have decided that Mongo DB is the best database to use for a web CMS. And this is for one reason: a query language.

Content is based on documents or content objects and this just cannot be modelled correctly in a traditional RDBMS like MySQL. Couch is great, but you gotta know what your content looks like up front (or write tons of code to create / drop indexes depending on content changes).

If I ever build the CMS that I would want to build - to allow users to create structured and unstructured content anywhere in the system - it would only work with Mongo.