Several folks have already said this, but it's worth repeating:
"Begin with the goal in mind"
* Define what "the best fighter" means to you. Only you can define this for yourself.
* Based on your definition, that would be the goal to strive for. As an example, is the "best fighter" a MMA champion, or an Olympic gold medalist? I think you could use elements of karate in either, but it would be different applications.
* Besides training in your art, work on flexibility, cardiovascular fitness, balance, and 'fast twitch' muscle strength. Having fast hands and feet helps, also.
* Mentally, immense self-discipline, strong willpower, adaptability, ability to predict what your opponent is doing. You may find it helps to be single minded.
* It may help to read biographies of people who were understood to be "the best" in anything. How did X become the best Y, and what lessons could I learn from them?
* Spiritually, you have to be able to get comfortable with being uncomfortable, develop self-control, and subordinate your present happiness for future greatness. You have to discipline yourself to your art.
* Even then, you probably need a lot of time, luck, great teachers, supportive family and friends.