In the beginning, we built most features on Ubuntu, but then they do not always live up to the highest quality, we have started to put everything in CentOS. CentOS is a free, community drive version of RedHat Enterprise solution. It picks all the packages from the very tested packages from the Red Hat Enterprise OS. This provides a very high quality and stability of the entire operating system. A very powerful foundation to build a web server on. Read more at https://www.centos.org/
As a web server, we have worked with both Apache and Nginx. Nginx is used today by all the 100 000 + largest web pages on the internet. It uses resources much better on a web server to be able to take care of very many visitors at once. Read more at https://en.wikipedia.org/wiki/Nginx
The database we use is MariaDB is a clone of MySQL (works identically to MySQL) but run by those who once started MySQL and then sold it to Oracle. With MariaDB can be absolutely sure that the database will continue to operate under free license. Moreover, the people behind MariaDB code improved a lot the past. Read more at https://en.wikipedia.org/wiki/MariaDB
PHP has seen major updates to the past and the new 7.x series has speeded up the PHP code with 20-40% on most web servers. There are also many new security enhancements that make your site much safer. There is no excuse not to be on PHP7 + today.