No-one knows which DI Container is the most popular one, but here's a tip:
Compare download statistics on NuGet. All the major .NET containers are available through NuGet, so doing that will give you an idea about the relative popularity ranking between the containers.
Be aware that this method is problematic from a statistical viewpoint for a number of reasons:
- There are other ways to download the containers. As an example, Unity also ships with Enterprise Library and can be downloaded from microsoft.com.
- Downloads don't equal usage.
- NuGet is (AFAIR) only available in Visual Studio 2010, so there may be a selection bias. Users of previous versions of Visual Studio may prefer different containers.
Despite the inaccuracy of the suggested comparison method, it's still the best that I'm aware of. Take it for what it is.