Something that's undisputed is widely accepted as being true. Does everyone in your class agree that you wear the snazziest, most fashionable clothes? Then you're the undisputed choice for Best Dressed.
Use the adjective undisputed when there's no question or disagreement. No matter how much your relatives argue at your family reunion, it's undisputed that your uncle's chocolate chip cookies are the absolute tastiest. And if a classmate accused of cheating admits to doing it, the accusation is undisputed. To dispute is to argue, and when you add the prefix un-, "not," you get something that can't be argued with.