Answers to a question about one's greatest achievement in context of a job interview can focus on beating sales or revenue goals, establishing new procedures or processes, developing ideas that changed the way things were done at the company, or winning new clients for a company. Answers to this commonly asked job interview question should be prepared before any interview.
As The Resume Center points out, while answers to a question about one's greatest achievements can cover accomplishments outside the work sphere, usually employers are looking for work-related answers. View this question as an opportunity to brag just a bit about specific accomplishments.
In preparing an answer regarding one's greatest achievement, stay away from overly cute or unprofessional answers. Prepare by brainstorming lists of accomplishments at recent jobs, including increasing sales, increasing customer satisfaction, and designing new scheduling procedures. Think as well about what former bosses may have said in person or during performance reviews. Prepare the answer by thinking about how the achievement came about, what obstacles had to be overcome, the effects of the achievement for the company, and the personal satisfaction or effects derived from the accomplishment. Make sure the example chosen has a demonstrably positive effect or involves an improvement in a procedure.