Is MariaDB the best solution for huge databases?

It's not just the database software used but how you use it, configure it and optimise it.

MariaDB 10.1, Percona 5.6/5.7 and Oracle MySQL 5.7 all have the potential to handle large databases extremely well. It all depends on how you configure it and what hardware you pair it up with that matters. Just using MariaDB 10.1, Percona 5.6/5.7 or Oracle MySQL 5.7 won't magically improve your performance without knowing how to properly optimise and tune your database configurations (assuming you've already had the database queries are optimised too).

Private paying clients I have worked with large databases, had anywhere from 50-400% improved performance just from me optimising and tuning their MySQL settings and that's on either MariaDB, Percona or Oracle MySQL servers

For instance for the average size server that is derived from Centmin Mod Survey 2016 survey, I would say most folks won't notice much difference if they were using MariaDB 5.5 or 10.0 or 10.1 with out of box default pre-optimised base default settings.