F#, first because its ecosystem is the whole .NET Framework world of libraries, utils, etc. Second, because it's got its own set of very functional libraries li

F#, first because its ecosystem is the whole .NET Framework world of libraries, utils, etc. Second, because it's got its own set of very functional libraries like suave.io for web apps/services, FSharp.Data for consuming data of diverse sources/natures, Fable.io an elegant JavaScript transpiler. Finally, with very demanding production projects like jet.com it already has proved that it can process massive amounts of transactions.