Avoid using something that the interviewer might consider questionable as a personal achievement. For example, saying your greatest personal achievement was doing keg stands in your fraternity or winning a beer pong contest would not impress your interviewer. While one of those might have been your greatest personal achievement, you should choose something that the interviewer will perceive as a great accomplishment and not immature or childish.

"The personal achievement I am most proud of is writing and publishing my first novel. It took me ten years of research and writing on the weekends when I had time, but I finally self-published it last year."