I want to develop a new website on magento 2. Should I use the latest officially released version of magento 2 which is v2.2.6 (nowadays) or is it better to use v2.3?
Magento 2.3 has many new features which are currently being tested and bug fixed.
The beta version has been launched so that the developers can start hands on the features. It is not for production websites yet.
There may be many small/big changes when the stable Magento 2.3 will be launched.
If you are planning to launch your website soon, I would recommend to with Magento's latest stable version (currently it is 2.2.6). If you want to try out the new features, you can install and start playing with it.
Magento 2.3 is now released If you are updating from an existing version follow this guide https://devdocs.magento.com/guides/v2.3/comp-mgr/bk-compman-upgrade-guide.html
otherwise if you are installing it brand new follow this guide https://devdocs.magento.com/guides/v2.3/install-gde/composer.html