Symfony

To say I've spent a lot of time working on Drupal 8 over the last 21 months would be a bit of an understatement. The Plugin System & the Blocks & Layouts Initiative have consumed much of my professional and personal time over that period, and we've worked on a lot of really awesome and interesting stuff. That being said, the vast vast majority of that work was still really "Drupal" and certain aspects of the underlying architecture that we were building on I didn't have the time to learn in detail.