F# is quite nice. I used it a long time ago for a couple of libraries when I was working in a team that mostly used C#. Fable looks really cool too.

I remember getting the feeling that C# is holding it back though. F# can't have any feature that it wants because interop in both directions seems to be a priority. If I recall correctly, that's what is holding back higher-kinded types in F#.

That, and the last time I tried F# on .NET Core things were still very rough. This was when .NET Core was still fairly new though. It might be fine now.