In your user profile you can choose 'When editing text' between 'Use standard web forms' and 'HTML' Editor.

But there is no option for the default input format when 'Use standard web forms' has been selected.

So if I prefere for example Markdown as my favorite input format I have to select the option againg and again whereever if write a text to a textarea.

A default input format setting in the user profile would be very nice.