Identity.NET for embedding a library, Identity Server for hosting a service.
They're not the simplest, and unfortunately the scaffolding story with Identity.NET is a bit of a nightmare for MVC apps (if you're not careful, you can end up with a ton of hidden-but-accessible razor page endpoints), but the code is robust and most cases are covered.
Identity.NET will also need you to write your own implementations of a couple of database access classes if you don't want to drag a dependency on Entity Framework Core along for the ride.
Documentation is fairly dry and mostly complete (Microsoft style) but be careful with versioning because dotnet core moves fast.
They're not the simplest, and unfortunately the scaffolding story with Identity.NET is a bit of a nightmare for MVC apps (if you're not careful, you can end up with a ton of hidden-but-accessible razor page endpoints), but the code is robust and most cases are covered.
Identity.NET will also need you to write your own implementations of a couple of database access classes if you don't want to drag a dependency on Entity Framework Core along for the ride.
Documentation is fairly dry and mostly complete (Microsoft style) but be careful with versioning because dotnet core moves fast.