We've used several different CI systems in the past four years, and Buildkite has been the best. Their configuration is highly flexible and the "bring your own infrastructure" aspect allows us to prioritize performance where it matters for us. We've gotten what used to be 12-18 minute builds running in under five minutes with layer caching and parallelization available in Buildkite.

The ability to ensure code stays within a controlled environment, even while using a SAAS platform.

Buildkite has a fantastic plugin architecture that lets me, my team, and even newbies-to-the-tool write their own plugins. Unlike Jenkins, these plugins don't break anything or interfere with the system in any way. Their support team is also amazing, super responsive and knowledgeable.