Theme templates with built in PHP loop

My personal recommendation would be Underscores: https://underscores.me/

Many other exist, such as Roots, HTML5Reset, HTML5Blank, etc. However, Underscores is arguably the most active and current most popular starter template for WordPress; heck, many of the official themes from WordPress are based on Underscores, too.

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Edit: Just searched around a bit, it seems that according to Konstantin Obenland, an employee at Automattic, every free and premium themes launched on WordPress.com by Automattic in 2013 and 2014 are based on Underscores.

Link to his interview: https://wptavern.com/introduction-to-underscores-a-wordpress-starter-theme-with-konstantin-obenland

You can find Konstatin Obenland listed on Automattic employee list here: https://automattic.com/about/