Regedit

This is one area where I personally would, out of principle, not trust any other application than Microsoft's own. More or less naturally, they're the only ones who can ever really know what they're doing here! In theory, at least. But the programmers working on Regedit are the ones who actually have access to the entire specification of how the registry works, and how its nature may change. So while, of course, they're not completely safe from making mistakes either, having the editor come from the same teams that define the format it edits gives a certain sense of trustworthiness.

Using any other application to access and modify your live registry is definitely a high-risk operation. Only do so if you're prepared to reinstall Windows if things go awry, because you can quite easily put your system past the help of System Restore.