Google's monorepo, and it's not even close - primarily for the tooling:
* Creating a mutable snapshot of the entire codebase takes a second or two.
* Builds are perfectly reproducible, and happen on build clusters. Entire C++ servers with hundreds of thousands of lines of code can be built from scratch in a minute or two tops.
* The build config language is really simple and concise.
* Code search across the entire codebase is instant.
* File history loads in an instant.
* Line-by-line blame loads in a few seconds.
* Nearly all files in supported languages have instant symbol lookup.
* There's a consistent style enforced by a shared culture, auto-linters, and presubmits.
* Shortcuts for deep-linking to a file/version/line make sharing code easy-peasy.
* A ton of presubmit checks ensure uniform code/test quality.
* Code reviews are required, and so is pairing tests with code changes.
* Creating a mutable snapshot of the entire codebase takes a second or two.
* Builds are perfectly reproducible, and happen on build clusters. Entire C++ servers with hundreds of thousands of lines of code can be built from scratch in a minute or two tops.
* The build config language is really simple and concise.
* Code search across the entire codebase is instant.
* File history loads in an instant.
* Line-by-line blame loads in a few seconds.
* Nearly all files in supported languages have instant symbol lookup.
* There's a consistent style enforced by a shared culture, auto-linters, and presubmits.
* Shortcuts for deep-linking to a file/version/line make sharing code easy-peasy.
* A ton of presubmit checks ensure uniform code/test quality.
* Code reviews are required, and so is pairing tests with code changes.