I would actually pick Vala over anything else for GTK3 applications. It’s really pleasant to work with and compiles to optimized C code, so you get serious performance wins over Python (and type safety to boot)
Vala really is a remarkable engineering achievement that is highly underrated. I can only imagine what could be possible if Vala had mindshare similar to Python.
Vala is nice, but is really niche at the time being. I think people would be better served using Python 3 and QT for maximum ease, documentation/help, and cross-platform abilities.