Numix-Circle has been a favorite of mine as long as it has been around. Now, I mostly use Breeze because it is highly compatible, beautiful, and widely used which makes it less jarring when I use other computers.

Back in the Plasma 4 days I joked that the default oxygen theme was ugly on purpose, so users would be forced to dive into customizing their own configuration (which is where KDE Plasma really shines above the rest). I think the defaults have come a long way, and it's nice to have a stock desktop ready-to-go without much customization.