There isn’t one universal “best,” but for a monorepo with automated builds and tests, the strongest default choices are usually:

Best overall for most monorepos: GitHub Actions

Why it’s a top pick:

• Great native fit if your code is already on GitHub
• Easy matrix builds and path-based triggers
• Good ecosystem and integrations
• Can be paired with self-hosted runners for scale and caching
• Works well for medium-sized monorepos without too much pipeline complexity

Best when: you want simplicity, wide adoption, and solid flexibility.

Best for large/complex monorepos: Buildkite or CircleCI

Buildkite

• Excellent for very large codebases
• Strong support for custom build workflows
• Easy to offload work to your own infra
• Powerful for distributed test execution

Best when: you need maximum control, fast parallelism, and enterprise-scale monorepo handling.

CircleCI

• Good caching and parallelism
• Easier setup than some more enterprise-heavy tools
• Strong build/test performance for many teams

Best when: you want speed and managed infrastructure with less operational overhead.

Best if you want fully integrated DevOps platform: GitLab CI

• Very capable monorepo support
• Strong pipelines-as-code
• Good artifact, registry, and permission integration
• Nice if you already use GitLab for repo, issues, and deployments

Best when: you want one platform for source control + CI/CD + security scanning.

Best self-hosted / control-heavy option: Jenkins

• Extremely flexible
• Huge plugin ecosystem
• Works anywhere, but requires more maintenance
• Can get messy in monorepos unless carefully standardized

Best when: you need full control and already have Jenkins expertise.

What matters most for monorepos

When choosing, prioritize:

• Path-aware execution: only run affected builds/tests
• Caching: dependency and build cache support
• Parallelism: distribute test suites efficiently
• Reusable pipelines: shared templates/workflows
• Artifact handling: promote only what changed
• Scalability: especially if many packages/services live in one repo

My practical recommendation

• If you’re on GitHub: start with GitHub Actions
• If monorepo is huge or test time is critical: choose Buildkite
• If you want an all-in-one platform: choose GitLab CI
• If you want simplest managed speed: consider CircleCI

If you want, I can also give you a feature-by-feature comparison table for GitHub Actions vs GitLab CI vs Buildkite vs CircleCI specifically for monorepos.