What is the best database for eCommerce?

I've worked in a huge eCommerce company.
MySQL or MariaDB to support the eCommerce front end.
PostgreSQL for sales interface.
Couchbase for unstructured data.

PostgresSQL

MongoDB

I'm much less inclined to mention products here, and more about having proper database management and DBA expertise available to support the database. You can use a technology that seems to match your workload best while still lacking in proper support, hardening, modelling and the like, resulting in a failed product and operational stability overall.

Furthermore, if you seek out an eCommerce software product, they will mention what databases they support - or even provide as part of the package, although that will get your DBAs to sigh and roll their eyes a bit, as such delivered database engines are often not sufficiently tailorable to the setup and support structure that your DBA team has in place.

Mongodb and PostgreSQL mostly works