First, you need loads of God given natural talent...the natural talent then needs slowly 'matured' through dedication...dedication to training and improvement...dedication to an outstanding work ethic...for a period in your life, nothing is more important than your goal to be the best you possibly can...dedication to sacrifice, you just can't always do what you want (or eat everything you want i.e. Riddick Bowe
...and obviously, you (like everyone) need to avoid drugs...and alcohol (plenty of time to drink when you retire in your 30's)...there were a lot of borderline great fighters who were sadly derailed by these things...
...then you need reasonable longevity...a 'prime' that lasts longer than 15 seconds...you need to have 'heart'...have to be able to take a punch, have to be able to 'weather the storm' after you've been really hit, have to be able to get the hell up off the canvas if/when you get dropped...you
have to come back from defeat, even the 'great' fighters lose fights...but the great ones come back unless old age is part of the reason...you need to be consistent in your performance and preparation...
...You must have some quality opposition...you need (or at least would like to have) a couple 'rivalries' that will bring out the best in both fighters...you need to successfully survive some 'wars'...If you dominate everyone you fight, you will be criticised for having 'weak', 'hand picked', or 'over the hill' opposition...you obviously shouldn't 'duck' competitive opponents...
...but most importantly...you need to win...and win...and win...and win...
...one last thing...it doesn't hurt to have a PR man and some positive media exposure...Kind of like the old question, 'if a tree falls in the forest and nobody is around, does it make a noise?'...If you're a great fighter, and nobody in the general public knows about you, you won't get the full advantages of it...you won't make the kind of money you deserve...the die hard boxing fans aren't enough to make a great fighter financially successful...You want to be remembered by people who know nothing about boxing!!!...that's not a bad thing...that's success!!!