The best for ".NET" is subjective. The only one that is built using .NET, RavenDB , but others, such as CouchDB and MongoDB also have “drivers” that allow them to be queried using a set of .NET libraries, so from the client they can all be used in a .NET application.
Although they all work on Windows as well, I think it’s more about the platform you plan to run. Check here for the language in which they are developed, it may be more difficult to install and maintain something developed in Erlang, in my opinion.
Another note is the way indexes are created. In RavenDB, you create indexes and map / reduce functions in .NET with Linq. In CoucbDB, you are using Javascript.
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