.NET Core is free, open-source, multi-platform framework from Microsoft. After years of Windows-bound .NET framework, it started to replace it few years ago and with .NET Core 5.0 it should replace it completely (target end of 2020). I use it regularly with C#, which is my favourite strong typed & object-oriented programming language.
Free, Open-source and Multi-Platform
It is designed to be run on multiple platforms (linux too).
I regularly check Web Framework Benchmarks on https://www.techempower.com/benchmarks/ and also https://benchmarksgame-team.pages.debian.net/benchmarksgame/. As far as I care, it always stands near the top.
Microsoft is heavily supporting the framework and constantly implementing new features into the language.
I keep an eye on machine learning (mostly I worked with it in Python). But this is open-source and cross-platform machine learning framework for .NET, definately check it here https://github.com/dotnet/machinelearning.