Xenko (formerly known as Paradox 3D) is an open source C# game engine. It is growing quickly and is supporting multiple platforms, with more to come.
Public roadmap visible at https://trello.com/b/FwbjOjjB/xenko-roadmap
Ranked in these QuestionsQuestion Ranking
Pro Good looking and fast
Looking nearly as good as Unreal Engine 4, but rendering significantly faster.
Pro Supports Visual Studio
Pro Straightforward editor
Paradox provides a simple and clean working environment for designing games.
Pro Supports Vulkan
Currently the only well known open-source game engine that supports Vulkan, and probably one of the only.
Pro Easy to learn and use C#
Pro No royalties or licensing fees
Con No user community
Unity and UE have a vast amount of user community.
Xenko does not.
Con Editor is Windows only
Since version 1.7, Linux can be targeted for runtime. The editor for the engine is available only to Windows though.
Con Install & build very dependent on specific versions of MSBuild
Install and project building are very dependent on specific and exacting versions of MSBuild.
The problem, of course, is that MSFT update Visual Studio quite a bit and even slight version number differences (for example installing now versus a week ago) seem to cause install and/or project build issues.
Con Documentation has not been updated
It is being updated since October, 2016 and if coming along fast.