What software dev skill has helped you the most?

I don't think it's a software skill specifically, but for me it was learning more formal methods for deconstructing and analyzing problems.

Learn version control!

git add .

git commit . -m Stuff

git push

That's All you need to get started. Then when you do something stupid you can just revert to a working version.

comment the natural language between your brain and code

Infinite patience with clients.