For me, branch protection rules have had the biggest impact.

Before using them, I would often push directly to the main branch. Now I create a feature branch for every change, open a pull request, and review the changes before merging. It has helped me catch mistakes, keep the commit history cleaner, and follow a workflow that's much closer to what I'd use in a team.

GitHub Actions is a close second. Even simple workflows, like automatically running tests or checking code formatting on every pull request, save time and give me confidence before merging.

One feature I wish I had started using sooner is GitHub Issues. I used to keep my task list in a text file, but linking issues to pull requests and commits makes it much easier to track progress.