I have created an MVVM library that I call TinyMvvm. I created it because I thought the other frameworks/libraries that I used before doesn't fulfill my requirements for an MVVM library for Xamarin.Forms. I have used it for a couple of projects now, both for my own apps and for apps for big enterprise customers. It would be really nice to get some feedback on it from other developers.
You find it on GitHub, https://github.com/TinyStuff/TinyMvvm
Nuget: https://www.nuget.org/packages/TinyMvvm.Forms/
I also wrote a blog post about it when I released 1.0 of the library, https://danielhindrikes.se/index.php/2018/06/26/tinymvvm-the-best-mvvm-library-for-xamarin-forms/