There are literally thousands of 'simple' themes to choose from and whatever I or anyone else considers 'best' or 'simple' WILL undoubtedly be different to what you consider 'best' or 'simple'.
SO;
FIRSTLY and MOST importantly, get over the idea that someone is going to
tell you what is 'best', only religions tell you what to think or what you have to know, use or do, and here are some methods to use for
you to choose what is '
best' for
you.
Method 1:
Open your WordPress dashboard, go to Appearance -> Themes -> Add New -> Feature Filter [set the options you want] -> Apply Filters (at the bottom) -> Install and activate that one.
Method 2:
Go to
https://wordpress.org/themes/ -> choose one -> download it -> upload to your installation -> activate it
Method 3:
Go to some website that has "free" themes -> choose one -> download it -> upload to your installation -> activate it
Method 4:
Go to some website that sells "premium" themes -> buy one -> download it -> upload to your installation -> activate it
When that is done:
REMOVE ALL THE DEFAULT AND OR UNUSED THEMES FROM YOUR INSTALLATION using FTP or your hosting control panel.
Install, activate and configure
this plugin to tighten up the WordPress 'security' [or lack thereof].