How to uniquely identify a website visitor?

I want to build an anonymous rating widget. Unauthenticated users can click "thumbs up" or "thumbs down" to express how they feel about something. I want people to be able to rate only one time, but I don't want to require login.

And so I want to be able to uniquely identify a visitor for the purpose of disallow double ratings. I am not interested in tracking them.

What is the best approach? Is using the IP good enough? Is a cookie or localStorage good enough? Should I use a combination of multiple techniques?

Since this is just a rating widget, I don't care much about security. If a user is hell-bent on circumventing my double rating detection, then I don't want to try too hard to stop him.