The best steel for chef knives typically lands between 55 and 60 on the Rockwell hardness scale - that’s hard enough to retain a sharp edge, but soft enough to avoid being overly brittle. (You want your knife to be able to take a beating without fracturing.) The harder a steel, the longer it will stay sharp but the more difficult it will be to re-sharpen on your own, often requiring professional service to get it back to its factory condition. If you’d like to learn more about their specific properties, the guide at KnivesAndTools is a great place to start.