I'm not talking ecosystem or which I'd choose to build an actual project with, just on a pure language basis, C#'s typing system is more flexible and less verbose than Java's, and unlike Java, C# actually treats functional programming as first class.

Java has certainly gotten better in both regards, but C# was really just a joy in comparison.

Well it's always about finding a good balance isn't it. Too many features like in C++ has negative consequences. Preferably you want something that lets you do all that you need to do, but not more. The trick to designing a good language is to let developers achieve as much as possible with as few features as possible, while keeping the code easy to reason about and understand.

This is obviously both subjective and highly dependent on what problem you are trying to solve, but I can't think of any situation in my career where C# would not have been a better a choice than Java from a strictly technical perspective. It's not just that the C# language is better, it's that the Java ecosystem is founded on poor design choices that result in code bloat and implicit behavior that is hard to troubleshoot and secure. See e.g. Spring, which automatically picks up and loads any logging library that happens to be in the user's path, even if that is an exploitable version of log4J. Java has become corrupted by enterprise architects. This satirical project demonstrates what I mean.

I say this as someone who is currently developing a FOSS Java library in my spare time, out of frustration with the Java code I had to endure at work.