In terms of maturity and feature-completeness, GTK# is the answer. Unfortunately, it's no beauty either.

Especially on Mac it does its own control rendering and, while it tries to match the OS, has problems.

XWT was meant to replace GTK# and has some good underpinnings but it doesn't seem super active right now.

As someone who tried to use WinForms on Mono on Linux, I'll save everyone who tries a few hours evaluation work: don't waste your time.

Text components in particular are very broken. Text selection, context menu, copy and paste, cursor movement, rendering, it's all very shaky.

Use Gtk# on Linux.