The license was explicitly chosen to enable code sharing with LLVM's libc++ (https://devblogs.microsoft.com/cppblog/open-sourcing-msvcs-s...).

The MSVC STL's quality is good enough for thousands of pieces of Windows software (including Windows itself & Microsoft's software such as Office) to depend and rely on. It delivers excellent performance for a broad range of use cases. It is actively developed in the open, delivering cutting-edge (C++23 & C++26) features, accepting Pull Requests and wonderfully documented on GitHub. It can be consumed using MSVC and LLVM clang-cl (which the MSVC STL maintainers test with CI infrastructure). The maintainers are actively working on "hardening" features to enable more secure C++ (https://github.com/microsoft/STL/wiki/STL-Hardening).

Unless you specify what "best" or "a library's quality" means to you, MSVC STL is excellent and because of that, the default choice on & for Windows.

Google chooses to only support libc++ for Chrome/Chromium (https://chromium.googlesource.com/chromium/src/+/main/docs/t...). libc++ is not a Google-owned project.