Output Custom Field in Drupal 8
{{ content.field_title }} 

This should work if you've got a field with that machine name.  If you're trying to print the title, the current syntax is:

{{ label }}


An easier way to figure out if the variables that you are trying to use is actually usable is to download the Devel module and enable Kint. Then in your .html.twig template file, you can enter:

{{ kint(content.field_myCustomfield) }}


and you would get an array with all the available variables.  If you don't want to install the devel module, you can just use:  I would recommend Devel/kint, it makes everything prettier and easier

{{ dump() }}


You're only going to need to dig into the array if you're trying to do something really custom. Eg: I used the following code in one of my projects to dig down into a link field so that i can output the exact url into an A tag inorder to wrap an entire region in a link.

&lt;a href="{{ content.field_card_custom_link[0]['#plain_text'] }}"&gt;


You can read more about outputting Variables in twig templates here:
https://sqndr.github.io/d8-theming-guide/twig/twig-debug.html

and on drupal.org here: https://www.drupal.org/theme-guide/8